( 710 ) 



not moved, tlic deflet-tictii of the ijal Nanometer also remained steady. 

 These observations also show how easy it is to measure an insu- 

 lating resistance with the string galvanometer. The experiment with 

 the ui-a]iium-ti'ioxide showed that the resistance of the layer of air 



u . . , .. , , . 60 Volts 



between the two |)lales of tlie condenser amounted to ~^y:rw—^^r~ — = 



^X 10-1^ Amp. 



= 1.2 X lt)'2 Ohms or rather more ^han a million Megohms. An 



insulathig resistance of 6X1^^'^ Ohms can be demonstrated wilh 



the 60 Volts laboratory -battery by a lasting deflection. 



We finally mention anothei- application of the string galvanometer 

 for measuring \ev\ feeble currents, namely those which are caused 

 by atmospheric electricity. A spirit-lamp is held up on a long pole 

 in the open air. An insulated wire connects the flame with one ter- 

 minal of the galvanometer-wire, the other terminal being earthed. 

 Under these conditions one sees a lasting deviation of the galvano- 

 meter which diminishes and disappears as soon as the pole is lowered 

 and carried indoors, but Nvhich i-eturns as soou as il is takeu out 

 and held up again. 



The deflection of the galvanometer in these experiments was 

 generally more or less oscillatijig on account of the wind causijig 

 fluctuations in the contact of the flame and the end of the wire. 



Besides for measuring feeble currents. Ihe wire-galvanometer is 

 suitai)le in practical work for detecting small quantifies of electricity 

 and especially foi- accurately measuring ra])id variations of electric 

 tension or of feeble electric current. As the instrument for feeble 

 currents which is (piickest in its indications, it will undoubtedly prove 

 useful for transoceanic telegraphy. 



The smallest quantity of electricity that can be detected by it. can 

 easily be calculated. Let us imagine that a great resistance has been 

 inserted in the circuit so that the electromagnetic damping of the 

 moving wire may be neglected and that now suddenly a current of 

 constant intensity is sent throuiiii the wire. 



The movement of the Avire under these circumstances is accurately 

 represented in the formerly published photograms ^). Theoretically the 

 wire will, at the moment the current starts, experience an electro- 

 magnetic force by which an acceleration will be imparted to it. Its 

 motion will be an accelerated one until a speed is attained such that 

 the resultant of the electromagnetic force and the tension of the wire 

 will make equihbrium with the resistance of the air. 



1) These Proc. June 27, 1903. p. 107, 



