CONTRIBUTIONS FROM THE JEFFERSON PHYSICAL 

 LABORATORY, HARVARD UNIVERSITY. 



THE THEORY OF BALLISTIC GALVANOMETERS OF 



LONG PERIOD. 



By B. Osgood Peirce. 



Presented November 11, 1908. Received December 22, 1908. 



If a ballistic galvanometer is to be used to measure the whole quan- 

 tity of electricity which flows impulsively in a circuit when a condenser 

 is discharged through it, or when the flux of magnetic induction through 

 the circuit is suddenly changed, it can generally be assumed that the 

 time during which the current lasts is so short that the flow practically 

 ceases before the suspended system of the instrument has moved sensi- 

 bly from its position of rest. That is, that the whole time of flow is 

 not greater than, say, one fiftieth part of the time required for the 

 needle or suspended coil to reach the end of its throw. 



It is often desirable, however, to determine with accuracy the 

 change of magnetic flux in a massive closed iron frame caused by a 

 given change of excitation, and in such a case it usually happens that 

 eddy currents in the metal or the inductance of the exciting coil so re- 

 tard the change that the process lasts for a number of seconds at least. 

 Under these circumstances a ballistic galvanometer of any ordinary 

 form is practically useless. Indeed, according to the experiences 1 of 

 Du Bois with such galvanometers as are to be found in most laborato- 

 ries, the ballistic method fails when the time required for the change 

 exceeds about one second. 



Slow flux changes can be measured, nevertheless, with the aid of 

 photographic records from a suitable oscillograph 2 either in the main 

 circuit of the magnet or in the circuit of a testing coil wound about 

 the iron. My experience with hundreds of such records seems to show, 



1 Du Bois, The Magnetic Circuit in Theory and Practice, Atkinson's transla- 

 tion, § 216, London, 1896. 



2 T. Gray, Phil. Trans., 184 (1893) ; Thornton, Electrical Engineer, 29 (1902) ; 

 Phil. Mag., 8 (1904); Electrician, 1903; Peirce, These Proceedings, 41 (1906); 

 43 (1907). 



