264 PROCEEDINGS OF THE AMERICAN ACADEMY. 



ably does so by blowing out the voltaic arc which tends to form. It may 

 be that the electrodynamic repulsion compels the oscillations of the spark 

 not to follow, so to speak, the voltaic arc and its current of heated air. 

 It seems as if the oscillations of the spark were true voltaic arcs, and that 

 the electrodynamic repulsion blows these out. There are, however, just 

 as many oscillations in the magnetic field as outside of it. The field 

 exerts no influence on the number of oscillations, or on their appai'ent 

 duration. The loud report which is produced when a spark is formed in 

 a magnetic field, notably when the primary circuit of an ordinary Ruhm- 

 korf coil is broken in a strong magnetic field, may indicate a sudden 

 stress in the medium ; in the case of the Crookes tube, the highly rare- 

 fied medium within it would effectually jirevent our hearing a similar 

 report. 



In order to see if the radiations from a Crookes tube emitting Ront- 

 gen rays could produce any effect upon the primary spark of the Thomson 

 Tesler transformer, I produced it near a Crookes tube, and examined it 

 by the method of damping. No change in resistance could be perceived, 

 and no eflFect was observed upon the length of the spark produced by the 

 secondary of such a transformer. In the next place, I resolved to deter- 

 mine whether differences in the materials of the spark gap made any 

 appreciable difference in the resistances observed in disruptive discharges. 

 I accordingly employed terminals of platinum, iron, aluminium, brass, 

 cadmium, zinc, and carbon. No difference arising from difference of 

 metals could be noticed. These experiments confirm the results obtained 

 by Rhigi * and by De La Rue and Hugo MuUer.f 



Moreover, no difference of resistance between spheres, between pointed 

 terminals, or between a point and a plaue^ could be perceived. With 

 powerful discharges such differences disappear. The employment of a 

 powerful storage battery together with a Plante rheostatic machine shows 

 conclusively that the dischai'ge in a Crookes tube, when on the point of 

 emitting the Rontgen rays most intensely, is an oscillatory one, and 

 that such discharge encounters a resistance less than five ohms. An 

 estimate of the great amount of energy thus developed in an exceedingly 

 small interval of time can be obtained if we suppose that Ohm's law holds 

 for individual oscillations. This reservation is an important one, for the 

 investigations I have described in this paper show that a discharge of 

 six inches in length encounters no more resistance during its oscillations 



* Nuovo Cimento, [2.], Vol. XVI. p. 97, 1876. 

 t Phil. Trans., Vol. CLXIX. Pt. 1, p. 93, 1878. 



