116 Alternating Current Magnetic Curve Tracer. 



centre of the I-magnetometer helix to the nearer pole, and 

 K' the distance from the centre of the I-magnetometer helix 

 to the farther pole. 



By knowing the distance of the compensating coil during 

 calibration from the I-magnetometer helix, its number of 

 turns, mean radius, and the current through it, we are enabled 

 to calculate its magnetic force at the I-magnetometer helix. 

 If the calibrating coil is a flat coil of only a few turns of wire 

 (as is always most convenient), then, as is well known, its 

 magnetic force at the I-magnetometer helix is expressed by : 



V - 27mCr2 . (9\ 



(cc) "~10(d* + r 2 )t ' ' * * ' ' [Z) 



where n is the number of turns in the calibrating coil, C 

 the value in absolute units of the current in it, r its mean 

 radius, and d the distance of the calibrating coil from the 

 I-magnetometer helix. Calling D e any particular vertical 

 deflexion in the course of an experiment, and D c the vertical 

 deflexion during calibration, we have, since the current in 

 the magnetometer system has been kept constant through the 

 whole experiment, the following relation : — 



5? -It 



(3) 



Substituting in equation (3) from equations (1) and (2) we 

 get 



D g SI (iff- Iff 8 ) 



D c tirn^W 1 



I0(d 2 -fr 2 / 



Solving equation (4) for I, we get 



(4) 



■ 10(rf» + r*)i SR*-R a \>l W 



We are thus enabled to find the value of the intensity of 

 magnetization of the sample in Absolute Units for any point 

 on the entire curve of magnetization. 



Knowing the number of turns, the length, and the current 

 flowing in the magnetizing coil surrounding the sample, 

 we can at once calculate in Absolute Units the magnetizing 

 force of the coil on the sample. In the calibration of the 

 horizontal deflexions, we know the current flowing in the H- 

 deflecting coils and the displacement it gives along the 

 horizontal axis. We calculate the magnetizing force of the 



