30 



Mr. T. Mather on the Calibration of 



But in the ordinary use of the instrument the line of zeros 

 is in the magnetic meridian, and when the needle is deflected 

 through angle the controlling couple is : : to sin 6. Hence 

 the current required to produce this deflection when 



the galvanometer is used in the ordinary way, is — — ^. 



- J ' sin D 

 For the currents must be proportional to the controlling 

 couples, since the relative positions of the needle and coils 

 are the same (fig. 2). 



Fig. 2. 



This being true for any deflection, the calibration-curve is 

 got by plotting and - — =~. 



Of course the same strength of current must be used in 

 determining the different values of D. 



To measure the angle D corresponding to any 0, we have 

 merely to break the circuit and observe the angle through 

 which the needle moves before coming to rest in the magnetic 

 meridian. is read off directly on the scale of the galvano- 

 meter when the current is passing. 



Below are appended the results of a calibration obtained by 

 this method, for which I have to thank Mr. Kilgour, a student 

 of the Central Institution, who made the observations : — 



e. 



D. 



sin 9 

 sinD' 



10 



34-8 



•304 



20 



353 



•592 



30 



32-2 



•938 



40 



29-2 



1-318 



50 



250 



1-813 



60 



19:2 



2-633 



70 



12-9 



4-209 



