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itself, ;tn effecl which always requires a certain time, and the 

 eolation of the plate is too rapid For the inductioi] exercised upon 



the glass bo |>lay ail appreciable pari. Besides, the production of 

 electrical layers of the same size upon the two faces of the move- 

 able plate is explained in a perfectly simple way by the action of 

 the points of the conductors and bhe armatures. The part played 

 by (lie apertures confirms also this explanation. 



A curious phenomenon connected with the Iloltz machine — in- 

 volving grave inconvenience when (lie machine is used for the 

 purpose of charging a large Leyden battery — must have been ob- 

 served by every one who has had much need to use the machine. 

 As soon as one of the armatures has been initially excited, the 

 Leyden jars connected with the two electrodes begin to be charged; 

 and after the rotation has been continued for a short time, sud- 

 denly the electricity is extinguished and resumed again in the 

 opposite direction, so that the positive electrode becomes negative 

 and vice versa. This change is detected at once by the change of 

 form of the glow at the points of the combs and the paper arma- 

 tures i each of the jars is then discharged slowly by the corre- 

 sponding comb, and becomes charged with the opposite electricity 

 to the same potential, after which the same inversion is repeated. 

 One of the best methods for getting rid of this inconvenience is 

 that of Iloltz himself. In this arrangement the paper armatures 

 are formed of two bands which traverse each nearly the whole 

 extent of a quadrant of the fixed plate. On the axis of the machine, 

 and just in front of the movable plate, is fixed a diametral con- 

 ductor terminated at each extremity by metallic combs similar to 

 ihe ordinary combs.' This conductor, which may be placed along 

 any diameter, is usually inclined at an angle of about 00° to the 

 direction of the two ordinary combs of the machine. According 

 to Prof. Mascart, the use of such a conductor, though it may 

 slightly weaken the power of the machine, entirely prevents both 

 inversion and extinction, so long as the movable plate is in motion, 

 even though the electrodes are widely separated. 



The last three chapters of the work are concerned with " Sources 

 of Electricity." First are discussed the experiments of Volta, 

 Thomson, &c. on the electrical separation produced by the contact 

 of dissimilar metals. The author, differing in this respect from 

 the French writers of a few years ago, entirely adopts the views 

 insisted on by the German electricians, and in this country by Sir 

 William Thomson and others. That is, Volta's original explana- 

 tion of his fundamental experiment is adopted, though at the same 

 time Volta's error, in supposing that there could be a flow of 

 electricity in a circuit composed entirely of metals all at the same 

 temperature, is pointed out. Some of Faraday's experimental 

 objections to Volta's theory are mentioned and speedily disposed 

 of, the author apparently deeming it unnecessary to discuss them 

 at any great length. 



Next come the experiments o]^ Ivohlrausch, Buff, Ilankel, &e« 

 on the difference of potential produced by the contact of dissimilar 



