Leakage of current

Good surface resistivity to prevent leakage of current across the surface of the material of the connector. 

The temperature coefficient of conductance is approximately 1-2 % per °C in aqueous 2> as well as nonaqueous solutions 27). This is due mainly to thetemper-ature coefficient of change in the solvent viscosity. Therefore temperature variations must be held well within 0.005 °C for precise data. In addition, the absolute temperature of the bath should be known to better than 0.01 °C by measurement with an accurate thermometer such as a calibrated platinum resistance thermometer. The thermostat bath medium should consist of a low dielectric constant material such as light paraffin oil. It has been shown 4) that errors of up to 0.5 % can be caused by use of water as a bath medium, probably because of capacitative leakage of current. 

A steady current does not flow in a perfect insulator in a static electric field, but energy is stored in the sample as a result of dielectric polarization. Thus, the insulator acts as a device to store energy. In reality, some leakage of current occurs even for the best insulators. 

If there is no leakage of current from the bridge network to ground, or from one part of the bridge to any other part, and there is no mutual inductance between the arms, Ii is equal to hy and /a to /4, so that... 

It has often been the practice in conductance work to ground certain parts of the bridge network for the purpose of improving the sharpness of the sound minimum in the detector at the balance point unless this is done with care it is liable to introduce errors because of the existence of leakage currents to earth. The telephone earpiece must, however, be at ground potential, otherwise the capacity between the telephone coils and the observer will result in a leakage of current. Other sources of leakage are introduced by the use of vacuum-tube oscillator and amplifier, and by various unbalanced capacities to earth, etc. 

Values of leakage current which are unusually high and which cannot be correlated to changes in volume resistivity may be caused by shunt error currents. The background leakage of current-measurement systems should be determined and monitored. 

FIGURE 4,3.3 An example of the cross-section of a piezoresistive cantiiever. It is important to cover the piezoresistor with a passivation layer, such as silicon nitride, to prevent leakage of current, especially in the case of measurements in a liquid environment. Reprinted from Ref. [24], with permission from Elsevier B.V. 

The coolant of fuel cells flows through a fuel cell stack and there is a danger of a short circuit of cells or a leakage of current if electrical conductivity of the coolant is high. Thus, ion exchanged water with an electrical conductivity of 1 mS or less, or a low-conductive antifreeze, is used as coolant. 

Surface resistivity is the leakage of current along the surface, expressed in ohms. It is also measured with a Wheatstone bridge and null detector, but with both electrodes on the same side of the sample. It is used to evaluate the effect of antistatic agents and suitability for service in charge dissipation. 

A bipolar system is sometimes used, in which a number of plates of impure copper, not connected to the electricity supply, are spaced out between the cathode and anode. Each acts as a bipolar electrode (q.v.), copper dissolving from one face while pure copper is deposited on the other, so that they eventually consist of pure metal. The energy requirement is much less for this than for the simple arrangement, but there is some leakage of current around the plates. Strict control is also necessary, as an insufficient passage of current would leave layers of impure metal, while any excess of current would be entirely wasted. 

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