Electrical Light round the Pole of an Electro-magnet. 465 



a vacuum is produced in it, having a tension of about 3 to 5 

 millims., is sometimes sufficient for the production of the phse- 

 nomenon ; but it is preferable to introduce this vapour directly, 

 and even a vapour arising from a more volatile liquid, such as 

 alcohol, sether, or sulphuret of carbon. Vapour of oil of turpen- 

 tine has also afforded me a good result. To introduce the vapoui's, 

 all that is necessaiy is to produce a vacuum in the balloon, and 

 to permit the entrance of air by opening it over a bottle contain- 

 ing the liquid the vapour of which is to be introduced, and which 

 is to be gently heated if, like oil of turpentine, it is not sufficiently 

 volatile ; a vacuum of about 4 or 6 millims. is then again to be 

 produced in the balloon. 



When the apparatus is thus arranged, the positive electrode 

 of the RuhmkorfF's apparatus is connected with the rod of 

 soft iron, and the negative one with the ring ; one or more 

 distinct luminous jets are immediately seen starting from the 

 extremity of the rod, and forming between this and the ring, 

 curved lines like those of the electrical egg ; at the same time 

 the upper part of the rod is covered with brilliant points, which 

 are agitated like the particles of a boiling liquid. As soon as 

 the rod of soft iron is magnetized by its being placed with its 

 base upon the pole of a strong electro-magnet, the luminous jets 

 acquire a rapid, very strongly-marked, and perfectly visible move- 

 ment of rotation in one direction or the other, according as the 

 pole of the electro-magnet is north or south. At the same time 

 the brilliant points which were upon the summit of the iron rod 

 disappear from thence and are driven to the edges, where they 

 fomi a luminous ring, which tui'ns like the jets and in the same 

 direction. When the direction of the induced discharges is 

 changed, that of the rotation is reversed. 



Not to dwell upon the different details of the phsenomenon, 

 which vary with the nature of the vapoiu-s introduced into the 

 balloon, a subject to which I shall return very shortly, I shall 

 confine myself at present to remarking that when the rotation 

 has continued a certain time the jets spread out, and at last form 

 around the cylinder of soft iron a cylindrical luminous and nearly 

 continuous coat which revolves with great rapidity, but of which 

 the rotation is often difficult to perceive, because of the conti- 

 nuity of the light. To reproduce the separate jets the rotation 

 of which is so distinct, the passage of the discharges must be 

 stopped and vapour again introduced. 



Without insisting afresh upon the analogy presented by the 

 luminous electro-magnetic pha;nomena which I have just de- 

 scribed with the aurora borcalis, an analogy which I have ex- 

 ])lained in my ' Treatise on Electricity,' vol. iii. p. 292, I can- 

 not, nevertheless, avoid taking this opportunity of recalling, in 



