Chapter 4: Electrodes

Polarizable versus Non-Polarizable Electrodes
Electrode Size and Class
Electrode-Electrolyte-Skin Junction
Impedance Checking
Unipolar and Bipolar
Noise
Silver / Silver Chloride Electrodes
Gold, Tin, Stainless Steel, Carbon Composition Electrodes
Electrode Gel Type and Ionic Content – Hypertonic, Hypotonic and Isotonic
Skin Preparation for Biopotential Measurements
Wet, Dry and Capacitively-coupled Surface Electrodes
Motion Artifact
Spot and Strip Electrodes
Equipotential Lines

Polarizable versus Non-Polarizable Electrodes

All electrodes fall into the range defined by two idealized types of electrodes, namely perfectly polarizable and perfectly non-polarizable. These electrode types are characterized by observing the consequence when current is passed through the electrode/electrolyte junction. Perfectly polarizable electrodes behave like capacitors, because only displacement (transient) current flows through the junction. A direct (non-transient) current does not flow through perfectly polarizable electrodes. In the case of perfectly non-polarizable electrodes, direct current easily flows through the electrode / electrolyte junction and requires no specific excitation voltage to permit the flow of electrons. Perfectly non-polarizable electrodes behave like resistors.

All electrodes have a half-cell potential that is measured in reference to the half cell potential of the Standard Hydrogen Electrode (SHE). The half-cell potential of the hydrogen electrode is arbitrarily defined as 0 volts. The half-cell potential, assuming no current is passing through the electrode, is known as the equilibrium potential (Ve). With a non-polarizable electrode, the potential of the electrode will not materially change from its potential at equilibrium (zero current state) even with relatively large currents passing through the electrode /electrolyte junction. This is because the electrode/electrolyte reactions occur quickly. With a polarizable electrode, the potential of the electrode will significantly change from its potential at equilibrium even with relatively small currents passing through the electrode /electrolyte junction. This is because the electrode /electrolyte reactions occur slowly.

Over-potential is the difference in the electrode /electrolyte potential of an electrode between its equilibrium and and operating states. An electrode is in operating state when a current is flowing. The over-potential consists of three elements:

Resistive Over-potential (Vr)

The additional potential that results from current flowing through the electrode / electrolyte junction due to resistance of that junction.

Activation Over-potential (Va)

The additional potential that results because of the difference in activation potential between two circumstances:

1. Activation energy barrier (voltage) required for a metal atom to oxidize and enter the electrolyte as a cation.
2. Activation energy barrier (voltage) required for a cation to be reduced and deposit a metal atom on the electrode.

Concentration Over-potential (Vc)

The additional potential that results because the concentration of ions at the electrode-electrolyte interface changes when current passes across through the electrode/electrolyte junction.

The total over-potential (Vp), where Vp = Vr + Va + Vc, represents the additional voltage needed to force the electrode reaction to proceed at the required rate. The operating potential of an anode is always more positive, with respect to cathode, than the equilibrium potential.

The total potential (Vt) associated with an operating electrode is its Equilibrium potential (Ve) summed with its Over-potential (Vp), where:

Vt = Ve + Vp

No electrode is perfectly polarizable or non-polarizable, however certain classes of electrodes can approximate these characteristics. Platinum electrodes are a reasonable approximation of perfectly polarizable electrodes and they exhibit Vp that primarily results from Vc and Va. Ag/AgCl electrodes behave reasonably closely to perfectly non-polarizable electrodes and they exhibit Vp that primarily results from Vr only. Generally considered, electrodes that are non-polarizable are used for recording biopotentials and electrodes that are polarizable are better suited for transient electrical stimulation. Polarizable electrodes can be used to record biopotentials, but because they behave capacitively, these electrodes are better suited for higher frequency biopotential measurements. Because non-polarizable electrodes behave resistively, they are better suited for biopotential recordings that range from high frequency to very low frequency.

Electrode Size and Class

Depending on the biopotential measurement of interest, differently-sized electrodes and classes may be required. Electrode size and class types are:

1. Micro-electrodes
2. Fine Wire Electrodes
3. Needle Electrodes
4. Surface Electrodes
5. Electrode Arrays

Micro-electrodes are typically saline-filled glass electrodes. These are pulled glass tubes that, when filled with saline, connect a very tiny exposed portion of saline to a conducting element. The glass tube is pulled, when held to a heat source, to craft a tapered tube. The conducting element, such as platinum wire or Ag/AgCl pellet, is held in the large portion of the tube. When the tube is filled with saline, the saline establishes a conductive pathway between the conductive element and the exposed saline at the electrode tip. The tip can be made so tiny as to be able, when placed against the surface of a cell, to connect to a small patch of the cellular membrane. This configuration permits measurement of cellular ionic channel activity in the patch area. Micro-electrodes can be used for intercellular or extracellular measurements. Intercellular measurements are recordings from a single cell and extracellular measurements will typically record signals from multiple cells.

Fine wire electrodes are single strands of metal wire – typically platinum or silver, that are partially insulated with a coating – typically epoxy. The insulating coat usually is applied to expose only an area at the tip of the wire. When a fine wire electrode is inserted into tissue, the metal conducting wire will be fully insulated from surrounding tissue except at the tip. Fine wire electrodes are too large to be used as intercellular electrodes. Fine wire electrodes are used as extracellular electrodes to collect, or introduce, signals from or to groups of cells or in specific tissue volumes. Fine wire electrodes can be made in a range of lengths and diameters, suitable for precision microscopic clamping systems to hand-insertion into tissue.

Needle electrodes are metal electrodes similar to fine wire electrodes. However, needle electrodes are usually much bigger and more robust than fine wire wires. Needle electrodes are usually made out of stainless steel, however a variety of metal alloys can be employed. Needle electrodes are usually partially insulated, like fine wire electrodes, however uninsulated needle electrodes are also used. When insulated, with an epoxy coat, the coating is applied to expose only an area at the tip of the needle. Needle electrodes are used as extracellular electrodes. Needle electrodes are typically hand-inserted into tissue.

Surface electrodes are typically the largest types of electrodes. These electrodes typically attach to the surface of the body via an adhesive tape ring or adhesive electrode gel. Surface electrodes come in a wide range of conductive materials, including metals, metal alloys, metal compounds and conductive rubber or fabric. The best performing surface electrodes for very low to high frequency biopotential recordings are Ag/AgCl electrodes that have an electrolyte-mediated connection to the body surface. Conductive rubber and other polarizable electrodes are better suited for electrical stimulation at the skin surface, versus biopotential measurements. However, for higher frequency biopotential measurements, all electrode material types can be used with varying degrees of success. A relatively new class of surface electrodes are active, non-contact, electrodes that employ electronics at the point of the electrode to establish a fully-capacitive connection to the body surface. These electrodes can have very good performance over a range of biopotential frequencies, however, they do not operate to zero frequency. Other new electrode types include stretchable, conductive, fabric electrodes that can be employed to collect biopotential data from ambulatory subjects.

Electrode arrays can range in size from microscopic to many square centimeters in area. Arrays can be arranged linearly, circularly or in a rectangular grid. Very tiny arrays can have imbedded electrode points of differing heights, able to be placed against the surface of a nerve fiber bundle where the different electrode point heights may intercept different fibers in the bundle.

Larger surface arrays can be placed against the skin to collect biopotentials over a selected area. A classical surface electrode array is the 10-20 EEG System. The “10-20” nomenclature refers to the inter-electrode spacing of 10% and 20%, depending on electrode location on the scalp. The electrodes are spaced either 10% or 20% of the full, front-to-back or right-to-left, circumferential distance of the skull. This system employs 19 active electrodes, each of which is normally referenced to the right ear, left ear or summed ears.

Electrode-Electrolyte-Skin Junction

Electrode-electrolyte-skin junction can be modeled as a cascaded series of resistance/capacitance networks and potential sources. The characteristics of these junctions are important to consider when measuring biopotential signals generated by the body, because these small signal currents must pass through the junctions to be measured. The resistances, capacitances and potentials associated with these junctions can change with the type of electrode and electrolyte or time, temperature and physical displacement.

The electrode-electrolyte junction fundamentally consists of a metal element in contact with an electrolyte. In this situation, an electron/metal ion /electrolyte ion interaction occurs. Metal ions enter the electrolyte and orient with respect to electrons in the metal and electrolyte ions in the electrolyte. This orientation layering results in a charge distribution (half-potential) at the metal-electrolyte junction. A simple circuit model for the electrode-electrolyte junction can be described as a resistance in parallel with a series resistance and capacitance, and the combined network is in series with a voltage source (half-potential). At low frequency, this impedance is mostly resistive and high-valued. At intermediate frequencies, this impedance is becomes more capacitively dominated and drops rapidly with increasing frequency. At high frequencies, this impedance becomes resistive and low-valued.

The electrolyte-skin junction is characterized by the the interaction of the electrolyte with the top layer of skin (epidermis). The epidermis, itself, is composed of layers. The top layer, the stratum corneum, consists of dead skin cells. These dead cells establish an impedance and behave as a semi-permeable barrier when in contact with an electrolyte. Ions underneath this barrier orient with respect to ions in the electrolyte, so a potential is developed. A simple circuit model for electrolyte-electrode skin junction can be described as a two resistances (electrolyte resistance and dermis resistance) in series with a parallel resistance and capacitance (from epidermis), and the combined network is in series with a voltage source (skin potential). At low frequency, this impedance is largely resistive and high-valued. At intermediate frequencies, this impedance is capacitively dominated and drops rapidly with increasing frequency. At high frequencies, this impedance becomes resistively dominated at low values.

The electrode-electrolyte-skin junction decreases in impedance with increasing electrode surface area. Conversely, very small skin electrodes will have high electrode-skin junction impedance. As electrode contact areas shrink, increasing demands are placed on the recording amplifier. This is because the recording amplifier input impedance should be much greater (100 times greater) than the electrode source impedance to minimize loading. As the source and load impedances increase, then the associated conductors running from the electrode-skin junction to the amplifier input become receptive targets for ambient displacement currents in the local environment. According, protective shielding is needed in these cases to control and redirect interfering displacement currents away from the sensitive input conductors.

Impedance Checking

Impedance checking between a pair of electrodes, attached to the skin surface, is a simple method to verify the conductive quality of electrode-skin junctions. For high-quality biopotential recordings it’s best to have low electrode-to-skin junction impedances. Generally, a good electrode-skin junction impedance measurement, consisting of the series combination of two, 1cm diameter, Ag/AgCl, surface electrodes and the tissue volume between, will be 10 Kohms or less. Nearly all the measured impedance is comprised by the two electrode-to-skin junctions (e.g. about 5Kohms each) and the tissue volume typically has much lower impedance (e.g. less than 100 ohms). This two terminal impedance measurement is best performed at a frequency which is mid-band in the spectra for the signal of interest. Surface biopotentials have most signal energy in the 1-300 Hz region. Consequently, anywhere from 10 to 30 Hz is a reasonable test frequency because a number in this range is roughly mid-logarithmic to the overall signal spectra amplified. Two terminal, electrode-to-skin, impedance checking using a DC current is unreliable because of the influences of the electrode /electrolyte equilibrium half-potentials, over-potentials, and skin potentials.

Unipolar and Bipolar

When electrodes are placed on a subject’s body, two electrodes are needed for any signal measurement. This is because a signal must have a reference against which the signal is measured. For many types of biopotential recordings, such as ECG, EEG and EMG, one reference might be used with many “active” signals. This type of biopotential measurement is called unipolar recording. When no global reference lead is used, the biopotential measurement is called bipolar recording

All biopotential measurements are intrinsically differential voltage measurements. This means that the amplifier is simply reporting the voltage sensed between two electrodes. A reference electrode is just one of these two electrodes considered as “reference”. And, automatically, the other electrode (in the differential pair) is considered as “active” or “input”.

Noise

All electrodes exhibit noise when attached to the skin surface. This noise has several components; a thermal component associated with the resistance of the electrode-skin junction, a “spiky” erratic component associated with sporadic ion transfer, a slow-moving “drift” associated with the offset potential between skin and electrode and “movement artifact” which may have “spiky” or “drift” aspects resulting from any electrode physical displacement on the skin surface.

Silver / Silver Chloride Electrodes

Silver / Silver Chloride (Ag/AgCl) electrodes are the best performing of all electrode types. The largest concern when using these electrodes is typically noise associated with movement artifact. Ag/AgCl electrodes are considered essentially non-polarizable. Non-polarizable means that the electrode / electrolyte junction will not develop an activation or concentration over-potential resulting from the flow of current through the electrode. This behavior creates a very stable electrode for the use in biopotential recordings. The AgCl layer, between the Ag layer and the electrolyte, has a stabilizing effect on the electrode / electrolyte junction, acting to reduce the noise in the junction, assuming that the Ag/AgCl electrode is in contact with an electrolyte that contains a sufficiently high concentration of Cl– ions. Sintered Ag/AgCl electrodes have the highest stability and plated Ag/AgCl electrodes perform nearly as well.

The half-cell potential of the Ag/AgCl electrode is usually estimated to be about 0.22 volts higher than the half cell potential of the hydrogen electrode, which has arbitrarily been defined as 0 volts. This half-cell potential will vary on the basis of the conducting electrolyte. If the electrolyte’s ionic concentration is saturated, the half-cell potential reduces to about 0.20 volts. When the ionic concentration is lower, as with seawater, the half-cell potential increases about 0.27 volts. When two Ag/AgCl electrodes are used to record biopotentials, the half-cell potentials cancel in the measurement loop, so the amplifier only records the biopotential signal evident in the tissue volume.

Gold, Tin, Stainless Steel, Carbon Composition Electrodes

Any metal, such as gold, silver, tin or stainless steel, can be used as a skin electrode. Gold and tin are often used for some kinds of biopotential recordings, typically EEG. Historically, Collodian (an adhesive and conductive electrode gel) has been used with tin or gold cup electrodes for high density EEG recordings. These electrode-gel configurations provide very low contact impedance to the scalp, for a small adhesive contact area, and are rugged for continual use.

Metal electrodes are polarizable. Polarizable means that the metal electrode – gel – skin junction will develop an offset potential over time, as current passes through the junction. This characteristic makes the electrode less useful for low frequency biopotential recordings (less than 0.1 Hz), because varying offset potentials can mask underlying slow biopotential signals.

Carbon Composition electrodes are flexible conductive electrodes consisting of carbon-impregnated rubber. These electrodes are used as stimulating electrodes, typically in transcutaneous nerve stimulation applications. These electrodes are very noisy and are not useful for general purpose biopotential recordings.

Electrode Gel Type and Ionic Content – Hypertonic, Hypotonic and Isotonic

For electrode gels (electrolytes), the higher the chloride salt content, the more conductive the electrode. Higher salt content, pre-gelled, surface electrodes are useful for making fast, high quality measurements of biopotentials, once the electrodes are applied to the skin surface. In addition, wet (liquid) gels further accelerate this process because the electrolyte migrates into the skin surface layers more easily and rapidly. High conductivity electrodes generally have reduced artifact, due to the low generated impedance between electrode and skin surface.

As the chloride salt content of the electrolyte drops, the less conductive the electrode. However, as the chloride content drops to 10% or less, then the electrode / electrolyte can be increasingly employed for long-term recording (greater than 2 hours), with reduced chance for skin irritation. In addition, if the electrolyte is encapsulated in a hydrogel, it’s gentler on the skin than wet (liquid) gels of the same salt concentration. Hydrogel-based electrolytes will not migrate into the skin surface as easily or rapidly as with wet gels. For 24 hour biopotential recordings, that do not require isotonic electrolytes, a 4%-5% chloride salt content hydrogel electrolyte is typically suitable.

The best performing electrodes employ a conductive, semi-fluid, compliant, layer between the skin and electrode fixed conductive element. This layer is typically a conductive gel. The gel is usually water-based and incorporates ionic content resulting from the addition of chloride salt. Hypertonic gel has ionic content greater than typical for the skin surface. Hypotonic gel has ionic content less than typical for the skin surface. Isotonic electrode gel has the same ionic concentration as the eccrine glands in the skin.

For Electrodermal activity measurements it’s important to use an electrode with similar (isotonic) chloride salt content as per the skin surface, so as not to hypersaturate or hyposaturate the eccrine glands. Skin sweat is a weak electrolyte and can be considered tonically equivalent to roughly a 0.3% (0.05 molar) chloride salt solution. Isotonic electrode gel use establishes a framework for non-invasive EDA measurements so these measurements will have minimal impact on the skin site chemistry. In practice, an electrolyte with a 0.1% to a 0.5% chloride salt concentration is typically suitable for EDA measurements, with concentrations in the 0.3% to 0.5% range as being somewhat more stable, when using Ag/AgCl electrodes, due to higher electrolyte Cl- ionic content.

Normal saline, which is isotonic to human plasma in reference to red blood cell membranes, is approximately 0.9% (0.153 molar) chloride salt solution. When exposed to normal saline, red blood cells do not shrink or swell in size.

The problem with isotonic and hypotonic electrode gels is that they result in high electrode contact impedance. This higher impedance can result in increased noise due to possible reduction in overall amplifier common mode rejection ratio and impacts of environmentally-sourced displacement currents. There will also be increased thermal noise due to the higher real part of the contact impedance. The impedance of the electrode/skin junction is highly dependent on the electrolyte type and the chloride salt concentration. For example, a hydrogel electrode with 4% chloride concentration will have about 10x higher impedance than a wet liquid gel electrode with 10% chloride concentration, after first application to the skin.

Hypertonic electrode gel is useful for obtaining high conductivity of the electrode-skin junction. A high conductivity electrode connection is helpful in biopotential recordings because lower contact impedance results in improved amplifier common mode rejection ratio, reduced impacts from environmentally-sourced displacement currents and reduced higher frequency noise. The use of hypertonic electrode gel may increase low frequency baseline drift, over the short-term, as ion transfer occurs through the electrode-skin junction. However, after a period of time (several minutes) the baseline will typically become more stable. Hypertonic electrode gel does not affect the quality of biopotential measurements that consist of high frequency information, such as general EMG, ECG or EEG signals. Moderately hypertonic electrode gel (Cl- molarity in the range of 0.5-2.2M), when used with Ag/AgCl electrodes, appears to provide stable electrode baseline potentials and improves measurement quality for all slow biopotential recordings, including the Skin Potential Level / Response, Electrogastrogram and Slow Cortical Potential.

The following reaction defines the requirement of Cl- in gels:

Ag + Cl- <=> AgCl + e-

A sufficient amount of chloride ions are needed, in the electrolyte, to maintain electrode total potential (Vt) stability. Generally, for biopotential recordings, an electrolyte chlorine ion molarity between 0.5 and 2.5 (mol/liter), coupled with Ag/AgCl electrodes, establishes the most stable electrode/electrolyte/skin surface configuration. Sintered (typically reusable) Ag/AgCl electrodes have the highest stability and plated (typically disposable) Ag/AgCl electrodes perform nearly as well.

NaCl Electrolyte Chemistry:

NaCl has one sodium atom and one chlorine atom per molecule. Sodium has an atomic weight of 22.99 grams/mole and chlorine has an atomic weight of 35.45 grams/mole. Accordingly, NaCl has a gram molecular weight of:

(1) * (22.99) + (1) * (35.45) = 58.44 grams/mole

Percent concentration (weight per volume) is the number of grams of chemical solute per 100 milliliters solution. Multiply percent concentration by 10 to obtain number of grams of solute in one liter of solution.

Examples:

A 5% NaCl solution will have (5 * 10) or 50 grams of NaCl in one liter of solution. To determine molarity (moles/liters) of solution, divide number of grams by gram molecular weight:

50 grams / (58.44 grams/mole) = 0.856 molar NaCl solution

A 0.3% NaCl solution will have (0.3 * 10) or 3 grams of NaCl in one liter of solution. To determine molarity (moles/liters) of solution, divide number of grams by gram molecular weight:

3 grams / (58.44 grams/mole) = 0.051 molar NaCl solution

To determine molarity of Cl- ions or Na+ ions, in solution, determine ratios of atoms to molecules. In the case of NaCl solutions, the molarity will be identical for NaCl, Na+ ions and Cl- ions, because there is one sodium atom and one chlorine atom in each sodium chloride molecule.

Skin Preparation for Biopotential Measurements

For highest electrode to skin conductivity, the skin should be lightly abraded with a gentle abrasive wipe, such as BIOPAC’s ELPAD. An alcohol wipe is not recommended, to improve conductivity, as this will only serve to dry out the skin surface. Lightly abrading the top layer of the epidermis will effectively remove dead skin cells and prepare the skin site to establish a high conductivity path, once the gelled electrode is applied.

After application, the electrode can be verified for robust galvanic connection to the skin via impedance checking. BIOPAC’s EL-CHECK can be used to measure the impedance between any two applied surface electrodes. Because each electrode/electrolyte junction forms a half-cell, impedance measurements are more accurately measured at some frequency resident in the band of biopotentials. EL-CHECK operates by injecting a 3.5uA rms constant current of 25Hz through the electrodes undergoing impedance check. The complete series impedance loop, including both electrodes/skin junction and coupling body impedance, is reported. Ideally, the reading should be 10,000 ohms or less (approximately 5000 ohms per electrode). In practice, BIOPAC biopotential amplifiers are very tolerant of electrode/skin impedances, even higher than 50,000 ohms. However, the highest quality recordings will always be accompanied by electrode/skin impedance junctions of 10,000 ohms or less.

Wet, Dry and Capacitively-coupled Surface Electrodes

Wet electrodes incorporate an electrolyte layer between the subject’s skin and the conductive electrode substrate (typically Ag/AgCl). These type of electrodes provide the lowest noise and highest signal transmission bandwidth. These electrodes are also optimal for measuring bioelectric potentials with very low frequency because they establish a direct current path to the signal source. Dry electrodes do not employ any electrolyte between conductive electrode and skin surfaces, other than skin sweat. Dry electrodes, if touching the skin directly, may provide both galvanic (direct-coupled) and capacitive (displacement current-coupled) electrical current pathways between the skin surface and electrode. A dry electrode that provides no direct-coupled current path, is considered capacitively-coupled only. A conductive material coated with an electrically insulating layer could be employed as a dry electrode, with no direct-coupled current path, and could function as a capacitively-coupled electrode even when placed directly on the skin surface. Capacitively-coupled electrodes can sense biopotentials some distance from the recording site. These electrodes operate via displacement currents only and so they have increasing inability to transfer signals of decreasing frequency. Because dry electrodes do not incorporate an electrolyte layer, they are easier to apply than wet electrodes.

Motion Artifact

Motion artifact includes the range of signals can can be produced during any motion, which act to mask signals of interest. Motion artifact has many sources, the following list indicates some commonly encountered artifacts:

1. Electrode-Electrolyte-Skin Surface Junctions

The multiple junctions in the electrode-electrolyte-skin interface all cause potentials. These potentials are sensitive to motion artifact. When an electrode is pushed against the skin, the potential changes can easily approach 1mV in magnitude. With electrode side to side motion against the skin, potentials of around 500uV are easily obtained. In addition, the electrode-electrolyte junction can produces artifacts when mechanically disturbed. An Ag/AgCl electrode will produce up to a 1.5mV signal when moved in an electrolyte. This potential can be reduced by eliminating motion of the electrolyte with respect to the Ag/AgCl electrode by placing the electrolyte /electrode junction at one end of a small cavity. An inert mesh is used to hold the gel still in this small cavity, next to the electrode.

2. Skin Potential Changes Related to Stretching

The top surface layer of the skin is slightly more negative (approximately 5mV) then the underlying layers. This potential is thought to be a junction potential between the electrolyte and underlying layers if the skin. Stretching the skin causes a reduction in the magnitude of this potential. If a surface electrode is placed on top of skin subject to stretching, the electrode will transmit this change of voltage. By slightly abrading the skin surface, this skin potential can be substantially reduced.

3. Triboelectric Effect

The triboelectric effect is the generation of an electrical charge as a consequence of friction between certain types of materials. Static electricity largely results from the triboelectric effect. In the context of motion artifact, triboelectric noise is the internal noise generated by the flexing or vibrating of a cable, which may be carrying a very small signal prior to being amplified. Cable movements can result in friction between the cable’s various conductors and insulators. In turn, this friction can generate triboelectric noise. This friction-induced electrical noise can easily surpass the magnitude of the signal of interest.

To reduce triboelectric noise in cabling systems, special low noise cable can be used. This cable will minimize the possibility of friction between the cable layers, employ materials that generate low triboelectric noise when in contact and incorporate conductive layers to drain away any triboelectric developed charges.

4. Faraday’s law, Hall effect and Lenz’s law

Faraday’s law:

Any change in the magnetic field environment of an electrical circuit will cause a electromotive force (voltage) to be generated in the circuit.

E (volts) = B (tesla) xL (meters) xV (meters/sec)

Hall effect:

When moving charges (current) travel in a conductor, perpendicular to a magnetic field, a voltage differential (Hall voltage) will develop across (transverse) to the current in the conductor.

Lenz’s law:

The induced emf in an electrical circuit, because of a change in a magnetic field, generates a current to oppose the change in the magnetic field. Accordingly, that current will result in a mechanical force opposing the motion of the magnetic field relative to the circuit.

In particular, in the context of motion artifact, any conductive circuit moving in a magnetic field will generate a voltage. The Earth’s magnetic field is approximately 50 uTesla. A 1 meter long conductor moving through this field at 1 meter/sec, in the orientation to maximally cut the lines of force, will result in a generated emf of 50 uV between the ends of the conductor.

5. Displacement Current Shifts

When a varying voltage difference is applied to two isolated conductors, a displacement current flows between the conductors. This principle is manifested clearly in the operation of a capacitor or transmission line. Given motion between conductors, displacement current magnitudes will change as the distance between conductors changes. The distance between conductors is inversely proportional to the effective capacitance between conductors.

In a typical laboratory environment, an isolated subject’s charge will be subject to the displacement current magnitudes and associated paths in the environment. Primarily, the displacement current magnitudes will be driven by local, high-level, varying voltage sources in the environment. Typically, the largest source is be the mains power network supplying electricity. Voltages would range from 120 VAC to 240 VAC and alternating frequencies from 50 Hz to 60 Hz.

As a subject moves around the environment, which such described alternating voltage (emf), this emf will induce an alternating charge on the subject. The alternating charge on the subject will vary in magnitude depending upon the flow of displacement currents surrounding the subject. As the subject moves closer to the source emf, all else being held the same, the charge on the subject will rise closer to the level of the source emf.

6. Magnetohydrodynamics

Magnetohydrodynamics is the branch of physics that is concerned with the behavior of electrically conductive fluids in magnetic fields. Observed behavior is largely subject to Faraday’s law and Lenz’s law. Magnetohydrodynamic (MHD) phenomena is very pronounced, and easily observed, when performing ECG measurements on a subject during magnetic resonance imaging procedures.

The influence of a static magnetic field on the blood flow inside the human vessel system leads to the MHD effect. This effect results in an additional voltage signal (Hall effect) that superimposes with the ECG signal. This superposition makes it impossible to perform a conventional diagnostic analysis of the ECG signal unless specific math is applied to the signal to compensate for the MHD effect.

The blood flow in the aorta is highest during systole, which corresponds to the ST segment in the ECG. Because of the Hall effect, the maximum MHD effect will result during the ST segment, thus elevating the T wave. In addition, absolute blood flow (stroke volume) will reduce in the presence of a magnetic field as per Lenz’s law.

Spot and Strip Electrodes

Spot electrodes are surface electrodes which typically establish a circular, or semi-circular contact area with the skin, via applied electrolyte. Strip electrodes make linear contact with the skin. In this later case, the contact line has a specific width and that width is established over a comparatively long distance. Strip electrodes are ideal for establishing equipotential lines to clearly identify specific measurement points in a volume conductor. The strip contact area is more conductive than the underlying tissue; so the strip forces all points of skin contact, along the circumferential strip, to the same potential. This type of electrode is useful for impedance cardiography and plethysmography measurements.

Equipotential Lines

An equipotential line is a line along the subject’s body which is required to be held at the same potential. In certain types of biopotential recordings, such as bioimpedance measurements, tissue volumes under investigation are typically bounded by equipotential lines. Typical bioimpedance measurements include impedance cardiography, impedance plethysmography and impedance pneumography.

In the case of impedance cardiography measurements, a constant and alternating current is introduced between the upper neck and lower torso, whereas the voltage monitoring electrodes are situated inside this bounded volume. Typically, the voltage monitoring electrodes are used to establish equipotential lines at the lower neck and middle torso.

A equipotential line can be established on the body surface of a subject by using strip or spot electrodes. In general, it’s more efficient and direct to use strip electrodes to establish an equipotential line because the electrode’s low impedance will automatically establish an equipotential line along the electrode/skin contact boundary. At least two spot electrodes are required to establish an circumferential equipotential line.