202 Prof. J. C. Poggeudorff on the Induction Apparatus 



nature of the points which maybe employed. The method con- 

 sists in the division of the current between two pairs of points 

 of diflcrent metals. The experiments (like the similar ones of 

 Faraday's with diflerent gases) involve some difficulties, and 

 require to be carefully interpreted ; because if by accident the 

 stream of sparks should be better conducted through one pair of 

 points than through the other, the former may, by the air be- 

 coming warm, and by the volatilization of metallic particles, 

 obtain an excess over the latter which is not natural to it. In 

 general, when the distances between points are equal, sparks 

 which are simultaneously obtained from both pairs of points 

 diiFer in colour and brilliancy according to the nature of the 

 metals, but apparently they do not differ in the spark-distance. 

 Very often the one or the other stream of sparks ceased for a 

 while without any very apjiarent cause ; on the other hand, how- 

 ever, when zinc, or still better tin, was compared with platinum, 

 it was always found that at soiall distances (1 or 2 millims.) 

 sparks only passed between the foruicr metals, at greater distances 

 (3 or 5 uiilHms.) only between the latter metal. Without doubt 

 at small distances the volatilized zinc or tin particles conduct so 

 well that they usurp the whole current. 



8. The luminous discharges m partial vacuum present similar 

 phsenomena to sparks in air. The increase of temperature 

 thereby is very considerable, although not so great as with 

 sparks. Although the strength of the current increases with 

 increasing rarefaction, the elevation of temperature decreases, 

 owing to the decreased resistance. 



9. In partial vacuum, too, the inequality in the temperature 

 of the poles is maintained, although it does not appear to be so 

 great as in free air. 



In one experiment the air was rarefied to about one inch of 

 mercury pressure, at which time, although the electric light was 

 continuous, it still produced a jingling noise, like that at the com- 

 mencement of the crackling of the sparks in free air. Two ther- 

 mometers stood at thedistance of about a line from the brass knobs 

 between which the discharge took place, and which were 1^ inch 

 asunder. During one minute the thermometer at the negative 

 pole rose 15° or 15 "5 C, the one at the positive pole 12° or 

 13^ C. 



All the above results were obtained by employing the thicker 

 and shorter of the described induction coils, a battery of two of 

 Grove's elements, and the condenser. 



