through Rarefied Air. 323 



tive, tlie iodine stain appeared five times under the above-defined 

 positive platinum point ; when the surface was negative, four 

 times under the other point. When the whole current of a mag- 

 neto-indudion apparatus is transmitted through very rarefied air, 

 between a very small and a comparatively great surface, the inter- 

 ruption current alone traverses the space between the surfaces lohen 

 the small one, in reference to that current, is jjositive ; when, on 

 the contrary, the small surface is negative with respect to the in- 

 terruption current, the contact current is also transmitted. 



It has long been known, that, on account of the small tension 

 possessed by the contact current, the interruption current is the 

 only one which can traverse a space of air. According to Poggen- 

 dorff*, this small tension of the contact current results from the 

 production of a closed conducting circuit in the neighbourhood 

 of the induction coil when the primary circuit is closed. Thus, 

 although M. Gaugain concluded from his experiments, that, with 

 a certain form of the electrodes, the interruption current did not 

 pass through veiy rarefied air, it follows from the present inves- 

 tigation, that in reality both the contact current and the inter- 

 ruption current are transmitted in this case. Consequently, if 

 the term valve is still applied to Gaugain's apparatus, it must be 

 considered as a valve for the contact current, and not for the 

 interruption curx'cnt. A glance at the luminous phsenomenon 

 furnishes a corroboration of the above theorem, if, indeed, any 

 were required. In order that the light might be more sharply 

 defined, I had the brass knob in the cylinder, as well as the disc, 

 stained black so as not essentially to injure the conduction at 

 their surfaces. When the small surface was positive, the lumi- 

 nous phsenomenon was the same as that which takes place be- 

 tween two equal knobs, and which has often been described. A 

 small, well-defined cone of reddish light extended from the small 

 surface to the negative electrode, and ended at some distance 

 therefrom, so that between the base of the cone and the negative 

 electrode a broad dark space remained. The whole surface of 

 the negative electrode, whether its form was that of a knob or a 

 disc, was surrounded by a beautiful blue light which stretched 

 also over the rod. The light was throughout equally bright and 

 quiescent. When, on the contrary, the small surface was nega- 

 tive, the light began immediately to flicker and change its in- 

 tensity. When the current had been previously regulated so as 

 to produce a very small deflection of the magnetic needle, or 

 none at all, the form of the light was much the same as before. 

 The reddish light of the positive electrode only was less distinctly 

 defined : it extended itself until it reached the negative electrode 

 without leaving a free dark space. The negative electrode itself 

 * Phil. Mag. vol. x. p. 124. 



