446 Messrs. Rowland and Hutchinson on the 



The needle system was entirely protected L id elec- 



trostatic effect. On reversing the electrificati lexions of 



from 5 to 7'5 millim. were obtained, after all p. utions had 

 been taken to guard against possible errors. Measurements 

 were made, and the deflexions as calculated and observed 

 agreed quite well ; but it was not possible to make the 

 measurements with as great accuracy as was desired, and 

 hence the present experiment. 



Helmholtz*, in 1875 and later, carried out some experi- 

 ments bearing on this subject. According to the "potential 

 theory " of electrodynamics which he wished to test, unclosed 

 circuits existed. The end of one of these open circuits would 

 exert an action on a close magnetic or electric circuit. So 

 the following experiment was made by M. Schiller f, under 

 his direction. 



A closed steel ring was uniformly magnetized, the magnetic 

 axis coinciding with the mean circle of the ring. This was 

 hung by a long fibre and placed in a closed metal case. A 

 point attached to a Holtz machine was fixed near the box, and 

 a brush-discharge was kept up from this point. If the point 

 acted as a current-end, a deflexion would be expected, on the 

 potential theory. No deflexion was observed, although the 

 calculated deflexion was 23 scale-divisions. The inference is 

 that either the potential theory is untrue, or else that there is 

 no unclosed circuit in this case, /. e. that the convection- 

 currents completing the circuit have an electromagnetic 

 effect. 



Schiller's further work, not bearing directly upon con- 

 vection-currents, leads him to the conclusion that all circuits 

 are closed, and that displacement-currents have an electro- 

 magnetic effect. 



Dr. Lecher is reported to have repeated Professor Rowland's 

 experiment, with negative results. His paper has not been 

 found. 



Rontgen % has discovered a similar action ; he rotates a 

 dielectric disk between the enlarged plates of a horizontal 

 condenser and gets a deflexion of his needle. He apparently 

 guards against the possibility of this being due to a charge on 

 his disk. A calculation of the force he measures shows it to 

 be almost one eighth of that in the Berlin experiment. His 

 apparatus is not symmetrically arranged, the disk being much 

 closer to the upper condenser-plate ; the distances from the 

 upper and lower plates are 0'14 and 0*25 centim. respectively. 



* Wiss. Abh. i. p. 778. 



t Pogff. Attn. clix. p. 456. 



j Sitzb. d. Berl. Akacl. Jan. 19, 1888. 



