221 



Notes ox the Calibration and Use of the Ballistic 

 Galvanometer. 



By C. M. Smith. 



The ballistic galvanometer is an important adjunct to an electrical 

 laboratory, inasmuch as it integrates the transient and varying currents 

 in the case of circuits which contain inductance or capacity or both. If 

 tlie time constant of the circuit is small compared with the quarter period 

 of the suspended system of the galvanometer, the first throw is propor 

 tional to the total charge which passes, orQ = yidt = G^. Where Q is the 

 charge, i/> is the observed first throw, and G is a constant expressed in 

 terms of coulombs or micro-coulombs per scale division. To interpret any 

 reading the galvanometer must be calibrated by passing through it a 

 known charge and observing the resulting first throw, the quotient giving 

 the value of G. 



This first throw is reduced somewhat by the so-called "damping," by 

 which is meant the effect of all those resisting forces which tend to absorb 

 the energy of a vibrating system, of any sort whatever. These forces are 

 generally assumed to be proportional to the velocity of the moving parts, 

 although there is no reason a priori why they should not depend upon other 

 functions of the velocity, as indeed they appear to do in some cases. How- 

 ever, long experience has shown that the simple proportion above stated 

 is a satisfactory generalization for slowly moving bodies, and one which 

 introduced into the general equations of motion leads to results quite in 

 accordance with experimental observations, for a large class of physical 

 problems. 



The earlier forms of ballistic galvanometer, now seldom seen in actual 

 service, were designed with small, highly polished needles of the Siemens 

 pattern, boll-shaped and slotted, and usually arranged much like the Kelvin 

 galvanometers of the same period, astatic, and highly sensitive. An essen- 

 tial feature, as pointed out in the older text-books, was that the damping 

 should be a minimum, in this tj^e of galvanometer being due to fiber vis- 

 cositv, air friction and the electi'o-magnetic reactions of induced currents, 



