152 



T=0,4 IK=2X10'" P= 1,0X10-'^ 



{HF) = 2o 000 D = 0,U K= 0,002. 

 'For a very sensitive instrument of the same kind: 



2'=0,Qd 1^::= 1,5x10^° P=3X10 5 

 (^i^) = 14 000 7) = 0,03 /i = 0,001 5. 

 The sensibility of this coil galvanometer with a time of indica- 

 tion of less than 2 seconds is greater than has ever been reached 

 even in the slowest instrument. It corresponds to a deviation for 1 

 microvolt of 6 mm. on a scale at 1 meter distance. 

 F'or a third, very quick but less sensitive specimen : 



7'= 0,12 ir= 2,8 X 10^" P= 0,45 X 10-^ 



(ffi^) = 18 000 D = OM K= 0,000 4.5. 

 K may be still further diminished, but then one has to face rather 

 great technical diflficulties, and soon the moment of inertia of the 

 mirror will play a too important role. For a mirror e.g. of a radius 

 of 2 mm. and a thickness of 0,2 mm. this amounts to 0,000 06. 



It may seem strange that this rather obvious measure of diminishing 

 the moment of inertia of the cod has not been made use of in the 

 construction of the coil galvanometer since long; the reason will 

 probably be a difficulty which arises when this principle is applied. 

 In the usual construction of the apparatus the coil is suspended 

 by a thin strip, which at the same time conveys the current 

 to be measured; the current is led off by means of a metal band 

 so slack as to have no appreciable share in the directive force 



of the coil. When now the moment of inertia of the coil is made 



« 



very small, and consequently its weight very slight, it becomes 

 subject to inevitable vibrations to a much higher degree, than is 

 the case with a heavy coil. This disadvantage may, however, 

 be obviated by not suspending the coil in the usual way, but stretching 

 the slack band. 



The reliability of the indication of a galvanometer with a coil 

 fixed in such a way appears conrincingly from the results of the 

 above-mentioned eclipse observations published elsewhere ^). 



Conclusion. 



In the construction of a coil galvanometer it is desirable to take the 

 moment of inertia of the coil much smaller than was usual hitherto ; 

 and not to suspend the coil, but to stretch it between two metal bands. 



Galvanometers constructed according to my directions are furnished 

 by the firm P. J. Kipp and Sons at Delft. 



1) W. H. Julius. Loc. cit. 



