the Electric Discharge in highly Rarefied Gaseous Media. 517 



reduced to 15° when the discharge was repelled against the outside 

 walls of the tube, assuming an apparent rotatory motion ; while, 

 on the other hand, it rose to 25° when the discharge was con- 

 densed against the inside walls of the spiral. It requires further 

 investigation to decide whether this effect of the direction of the 

 current and of the magnetization depends on the particular 

 form given to the discharge, or on the small diameter of the 

 tube in comparison with its length. 



§ 2. Experiments in which the electromagnet ivas placed in the 

 middle of the rarefied gas, inside the vessel in which the latter 

 was contained. 



I now pass on to the case in which the magnetic pole is within 

 the gas which transmits the discharge. In my first experiments 

 I used a glass globe, about 15 centims. in diameter, provided 

 w r ith four tubulures situated respectively at the ends of two diame- 

 ters of the globe perpendicular to each other. Two cylindrical 

 rods of soft iron were fixed, by means of two of these tubulures, 

 inside the globe, and in the direction of the same diameter, so 

 that their inner ends were about 8 or 10 centims. apart, while 

 their outer ends projected nearly 2 centims. from the tubulure. 

 In order to convert the internal ends into two magnetic poles, 

 the outer ends were put in contact with the poles of a strong 

 electromagnet. The other two tubulures served to introduce 

 into the inside of the globe two isolated metallic stems termi- 

 nated by balls, placed at about 10 centims. from each other, 

 which acted as electrodes for the electric discharge, the direction 

 of which was thus equatorial, or perpendicular to the straight 

 line joining the magnetic poles. As long as the soft-iron rods 

 are not magnetized, the electric discharge remains perfectly rec- 

 tilinear; but as soon as magnetization takes place the discharge, 

 which we will suppose horizontal, takes the form of a semicircle, 

 passing either above or below the line joining the magnetic 

 poles, according to the direction of the magnetization and to 

 that of the current. The form of the luminous arc is that of a 

 half ring, much flattened and widened ; the strise are very di- 

 stinctly marked in it, more so than they were in the straight dis- 

 charge ; and the outside is greatly indented, especially when the 

 gas contains a small quantity of water- or alcohol-vapour. If 

 the electric discharge, instead of being equatorial, is axial, that 

 is, directed from one of the magnetic poles to the other, the two 

 poles serving as electrodes, it does not undergo any sensible 

 change under the influence of the magnetization. 



Nevertheless, if the discharge is made to pass between a brass 

 ball and an iron ball placed on the end of an iron rod so that it 

 can be magnetized, the luminous atmosphere surrounding the 



Phil. Mag. S. 4. No. 226. Suppl. Vol. 33. 2 M 



