(711 ) 



\ï however the ieiisioii of the wire is feeltlo ciiongh, (he diii-aüoil 

 of this accelerated inotion is very small compared with the total 

 duration of the dellection so that it may be neglected. We are then 

 allowed to speak of an initial velocity of the wire and may disregard 

 its mass. The initial velocity is proportional to the current and may 

 be estimated at about 20 mm. per second for a current of 10~"^ Amp. 

 with an image as is obtained with our magnification ^). 



A current then of 10~^ Amp. only needs to last for — — sec. to 



cause a deviation of 0.1 mm. and as the photograms pro\e such a 

 deviation to be still visible, a quantity of electricity of 5 X 10-'- 

 Am[)ère-seconds can consequently be detected. This quantity is equal 

 to the charge of a condenser of 1 microfarad at a potential of 

 5 X 10-^ Volts or to the charge of a sphere of 4.5 cm. radius at 

 a potential of 1 V^olt. 



Since, as was pointed out above, the initial velocity is proportional 

 to the current, the deflection for a small quantity of electricity, will 

 entirel}' depend on that quantity itself, and it ^vill make no difference 

 whether a strong current passes during a short time or a feeble ciu'- 

 rent during a longer time, if only the time of passage be small enough. 



The properties of the wire-galvanometer lead us to expect another 

 very remarkable consequence, if the tension of the wire is increased, 

 the velocity with which a deflection is accomplished, will increase, 

 but at the same time the amount of the deflection for a given cur- 

 rent will diminish. Now it has already appeared from the photograms 

 that, provided the tension of the loire is not too (jreat, the change 

 in sensitiveness is exactly inversely proportional to the change in 

 deflectional velocity, so that the initial velocity for a given current 

 is independent of the tension of the wire. From this we derive the 

 seemingly paradoxical result that under the condition mentioned the 

 deviation for a quickly passed, small quantity of electricity is the 

 same for any tension of the wire. 



The facts are hi complete accordance with this argument and for 

 an observer who is not accustomed to the instrument, it is very 

 curious to see, how with a relatively nuich greater tension of the 

 wire and a consequent great diminution in sensitiveness for constant 



1) This amount of 20 mm. is only approximately true. I hope soon to be able 

 to deal more extensively with the movement of the wire under various conditions. 

 The influence of the viscosity of the air will tlien be compared with that of the 

 electromagnetic damping. It would be a decided advantage if the wire could be 

 placed in an air-tight space, which would enable us to observe its deflections either 

 in a vacuum or under increased pressure. 



