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Variation of the sensitivity of galvanometers. Some of the methods for reducing the sensitivity of galvanometers, which have been the subject of recent publications as well as some of the better methods described more than 10 years ago, but not generally known, are discussed, and it is shown that the latter are applicable under conditions not previously considered.
Series or Parallel Resistance. When the quantity being measured must be considered to be proportional to an electromotive force rather than to a current, the sensitivity may be reduced either by putting a resistance into the circuit in series or in parallel with the galvanometer. However, if a series resistance is used the resistance of the circuit, to an electromotive force developed within the galvanometer, is increased, while if a parallel resistance is used this resistance is decreased. Thus, not only is the damping changed, but if the electromotive force is transient or alternating the amount of resistance which must be used to bring about a specified reduction of the sensitivity cannot readily be determined since it depends in a complicated way upon the resistance of the source of the electromotive force and upon the constants of the galvanometer.
Series and Parallel Resistance. By the use of two series resistances and a parallel resistance the resistance of the circuit, considered both from the standpoint of the source of the electromotive force and from the standpoint of the galvanometer, may be kept constant while the sensitivity is reduced as much as may be desired. However, the damping may be kept constant and the sensitivity reduced in any desired ratio by means of a single series resistance and a parallel resistance. The parallel resistance may be connected either directly across the galvanometer or across the galvanometer and the series resistance. For the former arrangement
where X is the resistance of the source of the electromotive force, S is the series resistance, n the ratio of the normal sensitivity to the sensitivity desired, and P the parallel resistance. For the latter arrangement where T is the series resistance. These formulas are independent of the type and the constants of the galvanometer used, and with limitations which are considered apply whether the electromotive force is direct, impulsive, alternating, or transient.© 1925 Optical Society of America
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