Since the energy produced by an object may be small in relation to the time taken to produce it, it becomes necessary to utilize amplifier technique as in electrocardiography or electro-encephalography. With the appearance of better electronic equipment wider fields have now become capable of measurement in both clinical and experimental medicine. A survey of the physical principles and of various methods for measurements is presented below.To understand the electronic terminology used in this article the following definitions should be recalled.A difference in potential or voltage, E, (measured in volts) between two points of an electrical conductor will cause a current, I, (measured in amperes) between the two points. The current depends upon the resistance, R, (measured in ohms), of the conductor. This relationship is expressed in the formula (Ohm's law) :When the potential is constant, i.e., when the circuit has a constant polarity, designated by either + or -, the resultant current is a direct one (DC). When the potential is alternating, i.e., when there is shifting polarity, the current is alternating (AC). The number of polarity shifts per second is designated as the frequency (cps or Hz). A condenser can transmit only AC, against which it has a resistance inversely proportional to the frequency of the current and to the capacity of the condenser (measured in microfarad, MF). A coil (with or without an iron core) has a greater resistance to AC than to DC. The difference between the resistance to AC and DC depends on the inductance of the coil (measured in henrys, H). The resistance to AC increases with increasing frequency and with greater inductance.
