216 Mr. C. V. Boys on a 



seems that the calculated values for oxygen and nitrogen 

 are not very far wrong • but hydrogen is clearly incorrect. 

 The explanation of this anomaly is probably to be found in 

 the fact that the calculated molecular volume of hydrogen is 

 wrong, and that instead of being unity on our scale it ought 

 to be 3*5 like oxygen and nitrogen. In fact, the chemist 

 would infer that, as the difference in the complexity of the 

 molecular structure of hydrochloric acid, water, ammonia, and 

 marsh-gas does not affect the molecular volume under the 

 conditions we are discussing, in all probability the value for 

 hydrogen would be identical with that of the above-mentioned 

 bodies. If we adopt this view and change the value of the 

 T c -f-P c to 3 - 5, then the density of the fluid would become 

 0*034, which is in accordance with the experimental number 

 of Cailletet and Hautefeuille. An accurate determination of 

 the critical temperature and pressure of hydrogen, for which, 

 judging from the success of the experiments of M. Olzewski, 

 chemists will not have to wait long, will thus be of great 

 interest. 



XXIX. A Magneto-electric Phenomenon. By C. V. Boys, 

 A.R.S.M., Demonstrator of Physics at the Science Schools, 

 So u th Kensington * . 



EVERY one is familiar with the effect produced when a- 

 copper disk is set to spin in a powerful magnetic field : 

 the currents induced by the motion of the disk act in such a 

 direction as to oppose the motion, which therefore speedily 

 ceases. Faraday observed that if, instead of being set to spin, 

 a disk is merely suspended between the poles of an electro- 

 magnet, it will in general be disturbed whenever the current in 

 the coils of the electromagnet is made or broken. If it lie with 

 its plane parallel or at right angles to the lines of force, no 

 disturbance will be apparent if the lines of force where they 

 are included by the disk are parallel. But if the plane of the 

 disk makes an angle a with the parallel lines of force, then on 

 making the current in the electromagnet an impulse is given 

 tending to diminish the angle a, while breaking the current 

 gives an impulse tending to increase the angle «. 



Again, if the angle a be 90°, so as to eliminate this twisting 

 effect, no movement will be visible at the making or the break- 

 ing in a parallel field ; but if the disk be placed in a field with 

 diverging lines of force, in which, of course, the strength 

 diminishes as the lines separate, and if it be placed symme- 

 trically so as to include the greatest number (i. e. with its 



* Communicated by the Physical Society. Head June 28, 1884. 



