150 



Here a galvanometer is required wliicli is at the same time 

 sensitive and quick. 



In what follows a metliod will be discussed by which quickness 

 of indication may be reached for the sensitive coil galvanometer. 



Let us call K the moment of inertia of the coil ; D its directive 

 force ; T its semi-period of oscillation with open circuit ; F the 

 total area of its windings ; H the intensity of the uniform magnetic 

 field round the coil; W the resistance of the circuit, in which the 

 galvanometer is inserted; then the sensibility P of the galvanometer, 

 i.e. the angle of deviation (supposed small) for the unity of electro- 

 motive force in the circuit is given by : 



(HF) 

 F = ^- ^ (1) 



while 



'-^l^f ^^' 



Since we confine ourselves to the case of aperiodicity, we have 

 moreover, neglecting the air damping, 



(HFY 

 W^ — (3) 



and the time of indication is approximately given by t = 3 T. 



Six quantities occur in the three equations (1), (2), and (3), if we 

 count {HF), the so-called dynamic galvanometer constant, as one. 

 We can divide them into two groups, F, W and T being quantities, 

 which can be measured directly, and which characterize the instrument. 



The equations enable us to calculate from them D, K, and {HF). 

 Thus for the most sensitive galvanometer constructed by Siemens and 

 Halske according to Jaeger's directions : 



r=7,5 W=imohm = 10'' P=l,7 xlO-^i) 



are given, from which we calculate in round numbers : 

 {HF)=z 280 000 i) = 0,18 K=1,0. 

 For the quickest galvanometer of the same lirm, designed by 



DlESSELHORST : 



T=2 W=l,8x 10'' P=:0,6XlO-5 

 from which 



{HF) = 200 000 Z)=0,19 K=0,OS. 



1) This corresponds to a deviation of 3,4 mm. for 1 microvolt on a scale at 

 1 meter's distance. 



