AS A MEASURING INSTRUMENT. 



403 



when the coils lie in the magnetic meridian, as calculated from this 

 table, is as follows: 



These tables, in addition to what has been said, will sufficiently 

 explain the method. I will only remark here that the unit of force, 

 which in this table corresponds to a deflection of 36°, is, within cer- 

 tain limits, entirely arbitrary. Any current may be assumed which 

 admits of n in the first table being made zero, i. e., the coils being 

 brought into parallelism with the needle. By beginning with too 

 strong a current this would of course be rendered impossible. In 

 the example given above a rotation of 49£ was necessary to make 

 them parallel ; the limit of this rotation would of course be 90°. 



In constructing the first table, on which the second is based, it is 

 therefore necessary to make sure by a previous experiment that the 

 current used is not too strong to admit of the needle and the spiral 

 being placed parallel ; otherwise, the scale of intensity of the second 

 table could not be extended down to 0° of deflection. It is also best 

 to begin the values of m and n with this parallellism, that is, with 

 the zero value of n, and to continue the rotation from this point until 

 the value of n reaches 80°, or the point at which it is designed to 

 stop. 



A feeble current gives the most accurate results in the lower part 

 of the scale of intensity, and a stronger one in the upper ; two cur- 

 rents may therefore be used, if desired, in constructing the first 

 table. The stronger of the two need not give n = 0, but its strength 

 must be such that the smallest value of n which it gives shall coin- 

 cide with the greatest value of n from the other current, so as to 

 obtain a complete series of values of n from 0° to beyond 80°. 

 Moreover, the force of the current, in the scale of intensity obtained 

 in either way, can of course be reduced to the unit of either, and the 

 values not observed supplied by interpolation. 



The form of the curve which expresses the effect of the spiral on 

 the needle for the unit nf force is determined by the values of n as 



