270 



the circuit when one or both of these quantities is varying. Since that 

 meeting I have been successful in making use of an autographic device 

 for recording the variations of the current during its rise from zero to a 

 maximum immediately after the circuit is closed on a constant battery. 

 The e. m. f. is in this case constant, and the variation of the current indi- 

 cates the electro-magnetic inertia, and consequently, magnetic quality of 

 the iron. 



The autographic apparatus consists of a modiflcati«n of the Thomson 

 siphon recorder, used for submarine telegraphy. A rectangular coil of a 

 few turns is suspended between the poles of a powerful electro-magnet 

 which is separately excited. The coil is made to form part of the electric 

 circuit containing the magnetizing coil, and hence, as the current varies 

 in the circuit, the coil turns between the poles of the magnet. A long, 

 glass siphon pen records the motion of the coil, or rather its position at 

 every instant, on the record sheet of a chronograph. We thus get a curve 

 of which the abscissa? are intervals of time from the instant of closing 

 the circuit and the ordinate strength of the current at the end of these 



dc 



intervals of time. This curve must be expressed by the equation L—-+ 



RC=E where L is the coefficient of electro magnetic inertia of the cir- 

 cuit, R the resistance, C the current and E the impressed e. m. f. The 

 product EC represents the e. m. f. required to keep a constant current C 



flowing through the circuit, and L— represents the back e. m. f., due to 



dt 



self-induction. We may put the equation in the form L-—-^Y.—IiC=e, 

 or, Ldc=edt. The quantity e at any instant is the difference between the 

 ordinate at that instant and the maximim ordinate of the curve when 

 the impressed e. m. f. curve is drawn to such a scale as to coincide with 

 the maximum value of the current curve. We then get e readily from 

 the curve. Also edt is the increase of induction in time dt, and therefore 

 the area included between the current curve and the e. m. f. curve up to 

 any instant gives, when multiplied by the proper constant, the total in- 

 duction up to that instant. These different inductions, when expressed 

 graphically, with magneto-motive forces derived from the strength of the 

 current and the proper constants of the magnetizing coil as abscisste, give 

 the ordinary magnetization curve. From this curve the permeability of 

 the material, the dissipation of energy in cyclic action, and so forth, can 

 be readily derived in the ordinary way. 



