Abstract
The paper gives the theory of the action of a resonant shunt when used with an Einthoven string galvanometer, and records are given showing agreement with theory. In order to use a resonant shunt as a damping device, the following rules must be observed. The shunt constants must be so chosen that the free electrical oscillation in the shunt when short circuited is an exact copy of the free mechanical oscillation of the string when short circuited. This fixes the values of LC and of R/L for the shunt.
The absolute value of L depends upon the degree of damping required, small inductances yielding large dampings. For practically undamped strings, critical damping is obtained if the coil reactance at the resonant frequency of the string is equal to one-half the resistance of the string.
If the geometric mean between the coil reactance and the critical resistance of the string is equal to one-half the actual resistance of the string, the degree of damping is the same as if the galvanometer were on short circuit. In this case also the combination of galvanometer and shunt possesses no back electromotive force.