526 Prof. Magnus on the Changes in the Induced Current 



of the air-tube is too great for the intensity of the current which 

 this apparatus produces ; consequently we also find that the dis^ 

 charge no longer takes place in the shape of a bright luminous 

 line, but in a brush-like form. 



Not only are opposite currents produced if the resistance be 

 too great in proportion to the intensity of the current, but also 

 if the same be too small in relation to the discharge. 



If we choose such a separation of the polar wires in the air- 

 tube that the employment of the small induction-apparatus pro- 

 duces single currents, and if we then exchange the above 

 mentioned for the large coil, opposite currents are generated. 



We can obtain a similar result with one and the same induc- 

 tion-apparatus. If we attach the two ends of the spiral of 

 the large induction-apparatus to the test-tube, and likewise 

 insert the air-tube, in which the air is rarefied as much as pos- 

 sible, we see, when the polar wires of the latter are brought 

 sufficiently near together, that both are covered with intense 

 negative light. Let these wires be separated from one another, 

 and it will be found that the negative light on the positive wire 

 will gradually decrease, and increase on the negative wire, until 

 the positive wire is entirely devoid of light. Could the wires be 

 separated sufficiently, we should again have opposite currents ; 

 but the tube is not long enough for this. This result is obtained, 

 however, by gradually admitting air into the tube and thereby 

 increasing the resistance. 



The idea suggests itself that these opposite currents, formed 

 under so trifling a resistance, may have their origin therein — that 

 not only by the breaking, but also by the establishing of the cir- 

 cuit, a current may be induced. It is well known that Poggen- 

 dorff* has shown that, if the ends of an induction-coil are con- 

 nected by means of a metallic wire or a liquid which is a good con- 

 ductor, induced currents are formed on completing as well as on 



the induced current considerably more than the hammer used by Ruhm- 

 korff for his small apparatus does, which hammer by its own weight com- 

 pletes the circuit. The latter has, however, an irregular action. I hoped 

 by altering it in different ways to obtain a more regular movement, and for 

 this purpose I employed two plates of osmium-iridium in order to avoid 

 the adhesion, but I obtained no more favourable result. The contact- 

 breaker made for the large induction-coil is in any case preferable. It 

 likewise possesses the advantage, that we can accelerate its working at 

 pleasure by shortening the pendulum attached to it. It is not, however, 

 completely regular in its motion. 



A short time ago Riess constructed an apparatus by means of which the 

 breaking of the contact was brought about by clockwork — one of Malzel'a 

 metronomes. It is possible that it will have a more regular action in con- 

 sequence. 



* Poggendorffs Annalen, vol. xciv. p. 309. 



