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XLII. Rays of Positive Electricity. 

 By Sir J. J. Thomson, M.A., F.R.S* 



IN the experiments described in my previous papers on 

 this subject (Phil. Mag. Aug. 1907, Oct, 1908, Dec. 

 1909) the streams of positive electricity were produced by 

 means of a large induction-coil. When the discharges are 

 produced in this way the potential-difference between the 

 terminals varies considerably during the discharge, and the 

 method is not suitable for those experiments in which accurate 

 measurements of the potential-difference during the discharge 

 are essential. For example, I showed in my paper in the 

 Phil. Mag. for Dec. 1909, that the velocity of the most rapidly 

 moving particles in the positive rays is independent of the 

 potential-difference between the terminals and is equal to 

 about 2 x 10 8 cm./sec. A particle of the kind found in the 

 positive rays, if it moved with this velocity, would possess an 

 amount of kinetic energy equal to that acquired by the fall 

 of the charge through a potential-difference of about 20,000 

 volts. It is clearly a matter of importance to investigate 

 whether or not positive rays moving with this velocity can 

 be produced when the maximum difference of potential 

 between the terminals of the discharge-tube falls below this 

 value. To test this point satisfactorily fairly accurate measure- 

 ments of the potential-difference between the electrodes are 

 required. I have, therefore, obtained a Wehrsen (Mercedes) 

 electrostatic induction machine with two movable plates, 

 this furnishes a very constant discharge through the vacuum- 

 tube; the index of a B rami's electroscope whose terminals 

 are connected with the terminals of the discharge-tube 

 remains quite steady. 1 repeated with this machine the ex- 

 periments for which I had previously used an induction-coil, 

 and found exactly the same results. In particular I repeated 

 the experiments on the effect of the potential-difference on 

 the velocity of the Canalstrahlen, and again found that the 

 velocity of the swiftest rays is independent of the potential- 

 difference between the electrodes. 



With the Wehrsen machine I was able to measure the 

 potential-difference between the terminals of the discharge- 

 tube with an electrometer ; a method less open to objection 

 than that of measuring the length of the equivalent air-gap 

 placed in parallel with the discharge-tube. As a matter of 

 fact, however, I found by measuring the potential-difference 

 first by an electrometer and then by the air-gap method, 

 using the electrostatic machine to produce the discharge, 

 * Communicated by the Author. 



