151 



Not only does the above reasoning give us valuable information 

 about a given instrument, it also leads the way in the construction 

 of a new one, satisfying special demands. For tfie construction of a 

 galvanometer which is at the same time sensitive and quick, we have 

 to solve the problem how to choose B, K and {HF) so that P be 

 great and T small. 



Here we have to consider that the six quantities are related in 

 still other ways than expressed by the three equations. Indeed, the 

 mode of suspending the coil, the kind of wire of which its windings 

 consist, and its form will furnish additional relations between the six 

 quantities, and it is the question whether a certain change of D, 

 K and {HF), which would seem desirable according to the equations 

 (1), (2), and (3), is practically feasible. The equations will only indi- 

 cate the direction in which we have to seek. 



The quickness of the galvanometer only depends on the directive 

 force D and the moment of inertia R of the coil. In order to simplify the 

 calculation we will therefore leave the dynamic galvanometer-constant 

 {HF) out of consideration, and we do this the more readily as this 

 quantity may be modified within a wide range independently of the 

 five others. After the elimination of {HF) two equations remain : 



7' = jr / - (2) P^= (4) . 



We see from this that 7' is diminished when D is increased, but 

 that P is diminished in a corresponding degree. A second means of 

 increasing the quickness, namely diminishing K, will not alter 

 the sensibility so much, since K also occurs in the denominator of 

 the second member of (4). A third means, however, can lead to 

 our purpose. For a given value of W, P remains unaltered when 

 we leave 



~K~^ D}/D ""C^i^"ged. 

 Therefore by greatly decreasing K, say to one thousandth, and 

 at the same time diminishing F> to, say, one tenth, Twill be reduced 

 to one tenth, while P remains the same. 



That such a change of K and D is actually feasible, may appear 

 from the following data. 



For the coil galvanometer, by means of which the rapidly varying 

 radiation of the sun was registered during the central part of the 

 solar ecUpse of 1912 '), we have: 



1) W. H. Julius. The total solar radiation during the annular eclipse on April 

 17, 1912. Astroph. Journ. Vol. XXXVII, p. 225. 



