﻿272 Moving Coil and other kinds of Ballistic Galvanometers. 



following method has the great advantages of simplicity and 

 exactness ; it is not new: the writer learned it about ten 

 years ago from Prof. W. Stroud. 



Let l9 2 , #3, &c. he the successive swings of the galvano- 

 meter-needle. Then 



To find f observe, say, 1 and Z or 1 and 6- . 

 Then '/*-?.' or f*-* 



Also the corrected value of: the first swing is 



«v^ft(£JV*(£)*. 



In most cases the angular deflexions may be taken as pro- 

 portional to the deflexions on the scale. Let these be 8 lr 

 S 2 , S 3 , &c. Then the corrected value of the first swing is 



H|) ! »Ml)'- 



If the damping is small the corrected value is very nearly 

 $i + i (81 — 83), and this formula is sufficiently exact for most 

 purposes in practice. 



The following tahle contains a summary of the results 

 obtained : — 



Type of Galvanometer. Formula. 



(1) Tangent Galvanometer with small q _ HT^ 6 q_ Ti sin 6/2 



needle at centre of coil. Qtt 2 ' ir tan <p 



(2) Astatic Galvanometer with purely q_ Totfl q_ TiOcoscj) 



torsional control. ^ 27rM(G 1 +G 2 )' 2tt$ 



(3) Astatic Galvanometer with purely q_ T(M l H T cos « l +M 2 H 2 cos ec 2 ) ■ 9 



magnetic control. H ^(M^+M^Gj Sm 2 '*" 



= ?i^L^/ 2 



7r tan <f} 



(4) Moving-coil Galvanometerwith iron q __ Totfl Q T^ 



core and radial magnetic field. 27rC' — 2?r0' 



(5) Moving-coil Galvanometer with q_ Totfl q__TY0cos<£ 



narrow coil and no core. 27rC' iW) 



It will be seen that type (4) is superior to all the others as 

 regards the simplicity of the exact formula to be used with it. 

 It is also superior in most other respects. Galvanometers are 

 often met with in practice which strictly speaking do not belong 

 to any of the simple types considered in this paper. In such 

 cases the exact formulae are more complicated, and the best 

 plan to adopt is to determine experimentally the relation 

 between and Q. 



