Calibration of Galvanometers by a Constant Current. 29 



The operation might be continued for an indefinite period 

 were a sufficient supply of liquefied carbonic acid available, 

 but at Kew we usually find the 200-gallon bottle, which, 

 as already referred to, contains about 3 lb. weight of gas at 

 sufficient pressure to afford snow, adequate for our purpose. 



The cost of the materials is as follows : — gas 19s. 2d; ether 

 Is. 2d. 



We do not wish to claim any credit for the process above 

 described, but only to bring before the Fellows of the Society 

 a description of our modus operandi as a matter which may 

 perhaps be of a little interest to some of them. 



VI. Note on the Calibration of Galvanometers by a Constant 

 Current. By T. Mather, Assistant in Physical Department, 

 City and Guilds of London Central Institution*. 



THE principle of the method to be described consists in 

 measuring the turning moment exerted on the magnetic 

 needle of the galvanometer by a constant current, when the 

 needle and coils are in different relative positions. 



Suppose the galvanometer is placed in any position in a 

 uniform field, and that a suitable current be passed through 

 it, which current we will take as our unit current. The needle 

 will take up some position of equilibrium, say, at an angle 

 D to the magnetic meridian, and at angle 6 to the zero of the 

 scale (fig. 1). 



When in this position, the controlling couple is t: to 

 sin D ; and since this is balanced by the deflecting couple due 

 to unit current, we see that this couple due to unit current, 

 when the needle points to 6 is also : : to sin D (fig. 1). 



Fig. 1. 



>K 



Magnetic Meridian 



H^e 



* Communicated to the Physical Society by Professor W. E. Ayrton, 

 F.R.S. : read November 28, 1885. 



