Alternating Current Magnetic Curve Tracer. 115 



two-tenths of an ampere (obtained by means of a revolving 

 contact-maker on the shaft of the alternating-current generator 

 supplying the alternating magnetizing current) is passed 

 through the magnetometer helices, and kept accurately con- 

 stant throughout an entire experiment. For the continuous 

 magnetizing current is substituted an alternating current, 

 adjusted to such a mean value as will give the desired 

 maximum intensity of magnetization. As has been shown, 

 by varying the point in the revolution of the armature-shaft 

 at which contact is made, or current passes in the magneto- 

 meter helices, different deflexions will be obtained, the 

 rectangular components to which are strictly proportional to 

 the magnetizing force, and the intensity of magnetization 

 respectively, at the points in the alternating-current waves at 

 which the instantaneous direct current passes through the 

 magnetometer helices. Hence by varying the point of con- 

 tact on the revolving contact-maker, through a circular angle 

 corresponding to 360 degrees of phase in the alternating- 

 current waves, we have, as in the case of " static " cycles, the 

 successive positions on the screen of the intersection of the 

 cross-hairs in the reflected beam of light, marking points in 

 the outline of the energy-loss curve. 



§ 7. In calibrating the instrument for either " static " or 

 " periodic " cycles, the current through the magnetometer 

 helices is kept the same in kind and magnitude as during the 

 experiment. The sample is removed from the magnetizing 

 coil, and an accurately measured current sent through either 

 the magnetizing coil or the compensating coil, alone. The 

 deflexion is marked down on the vertical axis of coordinates. 

 Then an accurately measured current is sent through the H- 

 deflecting coils alone, and the resulting deflexion marked 

 down on the horizontal axis of coordinates. 



By knowing the distance of the resultant magnetic distri- 

 bution, or magnetic poles, from the I-magnetometer helix, 

 we can calculate, by well-known magnetometer laws, the value 

 of the magnetic force at the centre of the 1-magnetometer helix, 

 due to the long rod, in terms of the distance of the poles 

 from the centre of the helix, the cross section of the sample, 

 and the intensity of magnetization of the sample. It will be 

 plain, by considering the relative position of the long rod and 

 1-magnetometer coil, that this force, 



F/ r = SI^ — g-, 2 j; (1) 



where S is the cross section of the sample, I the intensity 

 of magnetization of the sample, R the distance from the 



12 



