PEIRCE. — BALLISTIC GALVANOMETERS OF LONG PERIOD. 



309 



about the core of the magnet depicted in Figure A. The curve OTJN 

 is a copy of the record of an oscillograph in the circuit when the elec- 

 tromotive force was suddenly applied at t = 0. The area between this 

 curve and its asymptote up to any value of the time represents the 

 whole change of the flux of magnetic induction through the coil, and 

 the difference between the ordinate of the asymptote and that of 

 the curve is proportional to the instantaneous rate of change of this 

 flux, and, therefore, to the induced electromotive force in a test loop 



Figure L. 



A portion of the record of an oscillograph in the circuit of a secondary coil 

 wound on the core of an electromagnet when the current in the exciting coil is 

 made to change by sudden steps in the determination of a hysteresis cycle. 



passed around the core. The general form of the induced current in 

 such a secondary circuit might be seen by looking at the curve just 

 mentioned upside down and through the paper. In this case the in- 

 duced current would practically come to an end in about five and one 

 half seconds. The line OZRXUPQN shows the growth of the main 

 current when there was an extra non-inductively wound resistance in 

 the circuit which was suddenly shunted out after about five and one 

 half seconds. Here, again, the general shape of the induced current in 

 the secondary circuit might be seen by looking at this line upside down, 

 from behind. The intensity of the induced current was inappreciable 

 after about eight seconds. 



Figure L shows the general shape of the induced currents in the cir- 

 cuit of a test coil of a few turns wound on the core of an electromagnet 

 when the current in the exciting circuit is made to grow by shunting 

 out a part of the resistance of this circuit by steps. If the currents, up 

 to the time OQ were sent through the coil of a long period ballistic 

 galvanometer, the resulting throw would not fall so much below the 

 throw due to the whole quantity of electricity carried by the currents, 

 sent instantaneously through the galvanometer at the origin of time, as 

 would the throw due to a steady current lasting for the time OQ and 

 carrying the same total amount. 



The examples already given will serve well enough to show what is 

 required of a galvanometer which shall measure accurately the whole 



