544 Prof. A. S. Mackenzie on the Deflexion of 



for the entry of wires to connect the plates with the Wims- 

 hurst machine employed for charging them. The greatest 

 difference of potential used was 10,000 volts, and the follow- 

 ing table gives the deflexions observed with that and lower 

 voltages : — 



Voltage between Observed deflexion 

 plates. to either side. 



Deflexion to either 



side reduced to that 



for 10,000 volts. 



10,000 



10,000 



8,000 



6,000 



6,000 



10,01)0 



"30 cm. 

 •30 „ 

 •24 „ 

 •20 „ 

 •20 „ 

 •31 ., 



•30 cm. 

 •30 „ 

 •30 „ 

 '33 „ 

 •33 „ 

 •31 „ 



Average deflexion d to either side produced by 

 10,000 volts = '31 cm. 



If we call F the electrostatic force along the path of the 

 rays in the plane midway between the plates, the value of 

 \dx ^Fdx was calculated graphically after the equations given 

 by Clerk-Maxwell in his ' Electricity and Magnetism/ 



§ 202. The value of calculated from the equation 



?-.lH 



FdxxW 



(see J. J. Thomson, ' Conduction of Electricity through 

 Gases,' p. 93), is 4'11 x 10 14 . The effect of the lines of force 

 beyond the plates is thus to give a value 8 per cent, larger 

 than if we had assumed that the field was bounded by the 

 plates and everywhere uniform. No account was taken of 

 the smaller effects due to the thickness of the plates, the 

 distortion of the field by the metal slit-tube, or to the fact 

 that the beam of rays is deflected from the central line, as 

 they are of the order of magnitude of the other errors 

 necessarily involved in the observations. 



The distance d was measured to the centre of the deflected 

 line, but the dispersion of the beam is not nearly so noticeable 

 as in the magnetic experiments. This is in part due to the 

 fact that the deflexions are smaller, and may be also in part 

 due to the exposures used, but I think it is safe to say that the 

 dispersion is not half so great as in the other case. Jn fig. 4 

 (PL VIII.) is shown a copy of a negative where 10,000 volts 



