Galvanometers of High Sensibility. 255 



Galvanometer Magnets. 



The problem usually at hand in the construction of highly 

 sensitive galvanometer systems is to construct a system which 

 shall have the greatest attainable sensibility for a given period 

 of vibration, as it is a great advantage to have a short working 

 period, especially where magnetic disturbances cause constant 

 variations in the zero. If we let 



m u = magnetic moment of the upper group of magnets, 

 m t = magnetic moment of the lower group of magnets, 

 M = m u + m % 



K = moment of inertia of the system (including also the non- 

 magnetic material), 

 H = strength of the resultant controlling field, 

 T = time of a complete vibration (undamped), 



then we have the following relations 



4tt 2 K 



T ,. 



(m tt — m ; )H 



But the deflection for a given current and set of coils. 



(m B + »? z ) 



or, substituting, 



d oc 



H(m u — m t ) 



(m„ + QT 2 

 4tt 2 K 



And for a given period this may be written 



7 M 



Consequently the greatest sensibility is attained by building 



M' 



up the system of such magnets that the relation -^ shall be a 



maximum. It was with this object in view of determining 

 the best dimensions of magnets, quality of steel, the mutual 

 action of magnets on one another when placed near together 

 as they are in galvanometer systems, the effect of boiling, and 

 of mechanical jars, the magnetic moments and inductions 

 attained in these short magnets, etc., that the experiments, 

 recorded in the following tables, were made. 



The magnetic moments of these small magnets were studied 

 by mounting a number of them, as nearly alike as possible and 

 whose dimensions had been carefully measured, on small brass 

 discs whose moments of inertia would be accurately computed 

 and were large in comparison with the moment of inertia of 

 the magnets, which entered as a slight correction term. The 



