( 258 ) 



internal resistance would be 5180 ohms and the normal sensitiveness 

 35 X 106. 



We will now discuss some other conditions by which the practi- 

 cabilit}' of the galvanometer for >arious purposes is determined. 

 Galvanometric methods may be di\ided into: 



I. Those in which an oscillating detlection is required. 



II. Those in which the detlection must preferably be aperiodic. 



The iirst methods are sul (divided into I A, those with a slow 

 period as in an ordinary ballistic gal\anon)eter, and I B, those with 

 a quick period, as in the optical telephone of Max Wien and the 

 vibration galvanometer of Rubens. 



The methods mentioned under I A are ap})lied for the measure- 

 ment of capacities and of small times by Pouillet's method, in general 

 always when small quantities of electricity have to be measured. 



Now the properties of the string galvanometer enable us to measure 

 these small quantities of electricity also with an aperiodic deflection. 

 If the electric current is only of short enough duration the deflection 

 of the string is in fact proportional to the quantity of electricity 

 passed. 



For the smallest (piantity of electricity which can still be demon- 

 strated we found on a former occasion ^) as the result of a rough 

 calculation 5 X iO~^- ampere-seconds, corresponding to the charge 

 of a sphere of 4.5 cm. radius at a potential of i volt. This calculation 

 was for a deflection of O.J mm. of sti'ing n". 10. For string n". 18 

 the actual measurement was made. The sensitiveness appeared to 

 be a little greater still: 1 mm. deflection for 4X10"^^ coulombs, 

 so that with this thread a quantity of 4 X 10 '^■- coulombs can be 

 demonstrated. 



But the sensitiveness for small quantities of electricity would still 

 be considerably increased if the damping of the motion of the string 

 could be i-emoved or diminished, e.g. by enclosing the string in a 

 vacuum. We should then obtain a slowly oscillating quartz thread 

 which would be thousands of times more sensitive than the most 

 sensitive ballistic galvanometers now existing. 



I B. The string gal xano meter can very Avell serve as an optical 

 telephone or as a \ibration galvanometer and so advantageously 

 replace the telephone as well for measurements of self-induction as 

 of electrolytic resistances. 



1) See these "Proceedings" G, p. /U7, l'JÜ4. 



