144 Dr. PI. A. Wilson on the 



It will be readily seen that the corrections for a gas4 

 thermometer when the pressure is kept constant and equal to 

 1 atmosphere are considerably larger than the corrections for 

 a thermometer employing the same gas on the constant- 

 volume method with an initial pressure of 1 atmosphere. We 

 also notice that two gas-thermometers constructed with two 

 different gases will differ in their readings more when used 

 according to the constant-pressure method with a constant 

 pressure of 1 atmosphere, than when used according to the 

 constant-volume method with an initial pressure of 1 atmo- 

 sphere. This result is quite in accordance with the experiments 

 of Regnault on the subject. 



VIII. On the Magnetic Effect of Electric Convection, and on 

 Rowland's and Cremieu s Experiments. By Harold A. 

 Wilson, D.Sc, M.Sc, B.A., Clerk -Maxwell /Student, 

 Cambridge University *. 



rjnRE object of this paper is chiefly to point out that 

 JL Cremieu's failure f to observe both the magnetic effect 

 of electric convection and its converse, the electrostatic 

 effect of magnetic convection, is to be attributed rather to 

 the methods employed than to the non-existence of these 

 effects. 



So many phenomena are known of which these effects 

 afford a simple explanation, that it is very difficult to believe 

 that their non-existence is possible. The magnetic deflexion 

 of cathode-rays and the Hall-effect in gaseous electrolytes 

 may be mentioned as examples of such phenomena. 



I shall first consider Cremieu's experiment for detecting 

 the electrostatic effect of a varying magnetic field. In this 

 experiment (fig. 1) a circular disk AB attached to a rectan- 

 gular frame ACDB was suspended by a wire E. The disk 

 surrounded a vertical bar-electromagnet NS, and was charged 

 by means of a battery through the wire E. During an expek 

 riment, the magnetizing current and the charge on the dis- 

 were alternately reversed ; and it was expected that the 

 electrostatic effect of reversing the magnet would cause the 

 charged disk to turn against the torsion of the suspending 

 wire E. No such rotation, however, was observed; and 

 Cremieu concluded that the effect in question does not exist. 



It is easy to calculate the impulsive couple on the disk 

 when the magnet is reversed. If dQ is an element of the 



* Communicated by Prof. J. J. Thomson. 



t Comptes JRendus, cxxx. pp. 1544-1549 (June 5, 1900) ; cxxxi. pp. 578- 

 581 (Oct. 1900), pp. 797-800 (Nov. 12, 1900). 



