CHAP, i.] ELECTRICITY AND MAGNETISM. 53 



Hence a very small initial charge is speedily raised to 

 a maximum, the limit being reached when the electrifi- 

 cation of the armatures is so great that the loss of 

 electricity at their surface equals the gain by convection 

 and induction. 



In the latest Holtz machines, a number of rotating 

 discs fixed upon one common axis are employed, and 

 the whole is enclosed in a glass case to prevent the 

 access of damp. A small disc of ebonite is now usually 

 fixed to the same axis, and provided with a rubber, in 

 order to keep up the initial charge which would otherwise 

 be dissipated. Holtz has even constructed a machine 

 with thirty -two plates, which yields very remarkable 

 results. Mascart has shown the interesting fact that 

 the Holtz machine is reversible in its action ; that is 

 to say, that if a continuous supply of the two electricities 

 (furnished by another machine) be communicated to the 

 two armatures, the movable plate will be thereby set in 

 rotation and will turn in an opposite sense. 



It has lately been shown that a Holtz machine can yield a 

 continuous current like a voltaic battery, the strength of the 

 current being nearly proportional to the velocity of rotation. It 

 was found that the electromotive-force of a machine was. equal 

 to that of 52,000 DanielPs cells, or nearly 53,000 volts, at all 

 speeds. The resistance when the machine made 120 revolutions 

 per minute was 2810 million ohms ; but only 646 million ohms 

 when making 450 revolutions per minute. 



LESSON VI. The Ley den Jar and other Acciimulators. 



47. It was shown in previous lessons that the opposite 

 kinds of electricity attract one another ; that electricity 

 cannot flow through glass ; and that yet electricity can 

 act across glass by induction. Two suspended pith- 

 balls, one electrified positively and the other negatively, 

 will attract one another across the intervening air. If 

 a plate of glass be put between them they will still 



