Sensitivity of a Galvanometer as a Function of Its Resistance (Classic Reprint)

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Excerpt from Sensitivity of a Galvanometer as a Function of Its ResistanceAn examination of Maxwell's analysis shows that the peculiar result he obtained (as given in equation depends upon his tacit assumption that t, the absolute thickness of the insulation, is constant for all sizes of wire. If in his preliminary equations t be taken as a constant fraction of D, the result obtained by Ayrton and Perry will be obtained, namely, that for maximum sensitivity the galvanometer should be wound to have a resistance equal to that of the external circuit.This theorem has often been quoted. For example, in the case of a Wheatstone bridge having a resistance of 100 ohms in each of the four arms the battery diagonal may be neglected for the condition of approximate balance, and, hence, the internal resistance of the bridge, which is the external resistance as far as the galvanometer is concerned, is 100 ohms. It is customary to say that in this case for maximum sensitivity one should choose a galvanometer having a 100-ohm coil if the choice lies between galvanometers which are identical in every respect except in size of wire, and, consequently, in number of turns and resistance of coil. While this advice can be followed with moving-magnet galvanometers, it will, in general, result in serious loss of time in taking readings if the galvanometers in question are of the moving-coil type without damping frames, because as a rule such galvanometers as now made are much over dam ed when connected to an external circuit having a resistance equa to the coil resistance. It will, however, be convenient, in the analysis 'now to be presented, to assume initially that the gal vanometers are either of the moving-magnet type, or that for other reasons considerations of damping may be neglected. The manner in which the conclusions reached may need tobe modified for movin coil [galvanometers will be indicated in a later section of this artic e entitled Damping Limitations of the moving-coil Galvanometer.About the PublisherForgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.comThis book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully, any imperfections that remain are intentionally left to preserve the state of such historical works.