521< Prof. Magnus on the Changes in the Induced Current 



the circuit of the induced current of either of the two induction- 

 apparatus, and if by a certain rarefaction and a certain separa- 

 tion of the ends of the two wires only single currents make their 

 appearance, then opposite currents are always formed if the ends 

 of the wires are separated to such an extent that the electricity 

 no longer passes between them in a luminous line, but spreads 

 itself out in a brush-like form. On a further separation of the 

 wires, opposite currents in every case show themselves in the 

 test-tube. Instead of moving the wires, the same result can be 

 obtained by gradually increasing the density of the air in the 

 tube ; in this case also, as soon as the brush-like discharge com- 

 mences in the tube, the currents begin to be in opposite directions. 



I believe it must be inferred from this that an increase of 

 resistance gives rise to the opposite currents ; and I have there- 

 fore employed the resistance of liquid and solid conductors 

 instead of that of air. For this purpose the air-tube was re- 

 placed by a glass tube 1 metre in length and 3 millims. in dia- 

 meter, in which two platinum wires could be approached towards, 

 or separated from one another at pleasure. If this tube be filled 

 with a saline solution, even if it contained only 025 per cent, of 

 sulphate of potash, and if the wires were 900 millims. apart, it 

 was impossible to obtain opposite currents. When, however, 

 the tube contained pure water, the result was like that obtained 

 by employing the air-tube. 13y a certain removal of the wires, 

 only single currents were produced, whereas by a further sepa- 

 ration they were opposite. It is likewise possible to produce 

 opposite currents by means of the resistance of metallic conduc- 

 tors ; it only needs for this purpose (provided, in addition to the 

 test-tube, no air- or water-tube is inserted) the spiral of the 

 large induction-coil, 40,000 metres in length, as a means of 

 resistance; for, on the production of currents by the small 

 induction-apparatus, they then make their appearance with great 

 distinctness. 



Further, if the resistance is increased in other ways, negative 

 light appears on both wires. If the sparks from the large induc- 

 tion-apparatus are allowed to traverse the air and then a test- 

 tube is inserted into the conducting wire, as long as the spark 

 traverses the air vigorously, negative light is seen at only one 

 pole of the test-tube ; but if the spark goes through the air with 

 a hissing noise, negative light appears at both poles. 



We likewise obtain, by the introduction of a thin plate of 

 mica into the circuit, which latter, with the exception of the test- 

 tube, consists of nothing but metallic conductors, negative light 

 on both wires. The same effect is produced, as Poggendorff* 



* Poggendprff's Annalen, vol. xciv. p. 326*. 



