1876.] Electro-Magnetic Rotation. 405 



But for producing the best effects, both of the rotating spark and also 

 of the spirals, there is a limit beyond which the exhaustion should not be 

 carried. At a pressure low enough to produce stratification, or even short 

 of it, the whole chamber is filled with the discharge, and all traces of 

 rotation and of spirals are obliterated. The stage best suited to the 

 purpose is that in which the discharge has thickened in diameter, and 

 where the spark proper has been replaced by a suffused light of the thick- 

 ness, say, of a quill. If the negative terminal be a surface (say the naked 

 surface of the soft iron pole of the electromagnet) instead of a point, 

 the sheet does not become contracted at the negative end, but remains 

 spread out and cuts the iron pole in a line radiating from the point. 



Various gases were tried — atmospheric air, carbonic acid, ether, chlo- 

 roform, coal-gas, hydrogen. Of these the first two succeeded best. "With 

 air the illumination of the flame-sheet was rather greater ; but with car- 

 bonic acid greater steadiness of position was obtained. With both ether 

 and chloroform occasional flashes, brilliantly illuminated, were seen j but 

 some chemical action appeared to take place militating against the steady 

 development of the flame-sheet. With coal-gas there was an inconvenient 

 deposit of carbon upon the sides of the chamber. With hydrogen the 

 cloud was not sufficiently developed. 



The success which attended the experiment with air may possibly be 

 partly due, as suggested above, to the combustion of the extraneous 

 matter floating therein ; and in fact the brilliancy and extent of the sheet 

 may be increased by attaching a piece of metallic sodium to the negative 

 terminal, or by causing a stream of any of the chlorides in powder, e. g. of 

 strontium, lithium, &c, to flow across the field of action. 



When a piece of sodium (or better still of soda) is attached to one of 

 the terminals, two effects may be noticed. When that terminal is negative 

 the whole of the flame is bright yellow, showing that the sodium is not 

 only detached but even carried across the field and deposited on the 

 positive terminal. When, however, the terminal to which the sodium is 

 attached is positive, it is found that the flame, when observed through a 

 red glass, appears yellow to a certain distance from that terminal, but red 

 beyond, and also that the pitch of the helix is less near the positive than 

 near the negative terminal. These effects may be attributed to the pre- 

 sence of metallic vapour evolved by the heat at the positive terminal, but 

 not carried across the field as when the terminal in question is negative. 



The following explanation of the phenomenon is due to Prof. Stokes, 

 from whose correspondence it is substantially taken. The mathematical 

 solution, although only roughly approximate, is perhaps still quite suffi- 

 cient to give the general character of the experimental results. 



The magnetic field will be supposed uniform, and the lines of force 

 parallel straight lines from pole to pole. The path of the current when 

 undisturbed is also a straight line from pole to pole. In such a condition 

 of things, every thing being symmetrical, no rotation would take place. 



