Chemistry and Physics. 391 



8. Electrical tension at the poles of an induction apparatus. — 

 The electromotive force necessary to produce a spark of a certain 

 length has been variously stated by investigators. Heydweiller 

 (Wied. Ann., 48, p. 313, 1893) calls in question Professor Elihu 

 Thompson's statement that a striking distance of 80 cm requires a 

 potential difference of about 500,000 volts, and thinks that this 

 is a very great overstatement and that 100,000 volts would be 

 nearer the truth. A. OBEEBECxhas begun an investigation of the 

 subject and points out that the maximum rise of the curve of 

 electromotive force in an induction coil produced by interrupting 

 the primary circuit should be the starting point in an investiga- 

 tion of this subject. Oberbeck finds that a potential difference of 

 60,000 volts can produce under certain conditions a stream of 

 sparks of more than 10 cm in length. 



Experiments in the Jefferson Physical Laboratory with a stor- 

 age battery of 10,000 cells connected to a Plant e rheostatic 

 machine lead the author of this note to conclude that Professor 

 Thompson's estimate is nearer the truth than that of Heydweiller. 



— Wied. Ann., No. 0, 1897, pp. 109-133. j. t. 



9. Investigation of the Lenard rays. — Th. des Cotjdres 

 describes a simple method of studying cathode rays in free air. 

 He employs a small transformer for exciting the rarified tubes, 

 the primary of which consists of only three turns of a band of 

 copper, while the secondary consists of about sixty turns. The 

 primary is excited by the discharge of a Leyden jar. The con- 

 struction of suitable tubes with aluminum windows is fully 

 described. The author concludes that cathode rays behave in 

 the outer air precisely as they do in the rarified space inside the 

 tubes. — Wied. Ann., No. 9, 1897, pp. 134-144. j. t. 



10. Behavior of rarified gases in approximately closed metallic 

 receptacles in a high frequency field. — Faraday showed that no 

 electricity could be perceived inside a metallic cage, the exterior 

 of which was connected with the ground. H. Ebert and E. 

 Wiedemann show that this conclusion is not correct when the 

 space inside the cage is filled with a rarified gas, which is sub- 

 jected to the oscillations of an electric field. They conclude that 

 in order that an electric charge shall penetrate through the holes of 

 a metallic net into a space surrounded by this net, it is necessary 

 that a rarified gas should exist on both sides of the net. The 

 lighted gas apparently conducts the energy into the inner space. 



— Wied. Ann., No. 9,"l897, pp. 187-191. " j. t. 



11. Cathode Rays. — In a very important paper Professor J. J. 

 Thomson discusses the ether theory of the cathode rays and the 

 electrified particle theory. The latter theory has the great 

 advantage that it is definite and its consequences can be predicted, 

 whereas the ether theory depends upon unobserved phenomena in 

 a ether the existence of which is in doubt. Thomson shows by 

 ingenious experiments that the cathode rays carry a charge of 

 negative electricity, and that they are deflected by an electro- 

 static field. Since they are also deflected by a magnetic field, he 



Am. Jour. Sot.— Fourth Series, Vol. IY, No. 23.— Nov., 1897. 



27 



