183 
1908-9.| An Improved Form of Magnetometer. 
in the north and south direction, and it is evident from the figure that if 
H is the horizontal component of the earth’s magnetic field at P, the total 
directive force at the needle is H + (F s sin + F c sin 0 2 ). If the current is 
reversed in the circuit the directions of F s and F c change, and the directive 
force at the needle becomes H - (F s sin d 1 + F c sin 0 2 ). 
The presence of the effect referred to may be made apparent by placing 
a permanent magnet close to the magnetometer, and thus deflecting the 
needle. On reversing a current in the circuit a change in the deflection 
will in general be observed. The magnitude of the errors introduced may 
be determined in this way for various parts of the scale and allowed for 
in the results, or the coils may be rotated until the effect disappears. If 
the former method is adopted, the labour of computing the results is much 
increased, and, further, it is difficult to make a proper correction, since the 
allowance to be made is a function both of the angle of deflection and the 
strength of the current. The second method can only be used if the coils 
are capable of being rotated on their stands, and the adjustments would be 
difficult and troublesome to carry out. 
The necessity for attending to this source of inaccuracy was first pointed 
out by Erhard,* who investigated the magnitude of the errors which were 
involved by neglecting it. In the case of a magnetometer of the usual 
type examined by him, it was found that with a magnetising field of 
128'3 C.G.S. units in the solenoid there was a change of 6*8 per cent, in the 
directive force on the needle on reversal of the current. Erhard advised that 
the magnitudes of the errors introduced should be determined for various 
parts of the scale and allowed for in the results. 
While carrying out a research on certain feebly magnetic alloys the 
authors found that the elimination of the aforementioned sources of error 
caused very considerable delay in the progress of the work. An attempt 
* “ Eine Fehlerquelle bei magnetometrisclien Messungen,” Ann. der Phys ., 1902, p. 724. 
