594 HISTORY OF SCIENCE. 



namely, the one invented byHoltz of Berlin, 1865. It consists prin- 

 cipally of two circular plates of glass, of which one, A, is fixed and 

 supported on insulators at four points. The other, B, is somewhat 

 smaller, and capable of receiving a rapid rotation from the multiplying- 

 wheels s s, s' s'. In the fixed plate are two large holes, a and b. Ad- 

 joining these on the glass are two bands of paper, c and d, provided 

 with pointed pieces which project into the opening in the opposite 

 direction to that in which the plate B revolves. At certain positions 

 opposite the movable plate are fixed the metallic comb-like rods gg 

 and //, // and vv. When it is desired to set the machine in action, 

 it is started by placing the knobs p and n in contact, and communi- 

 cating a small charge of negative electricity to the paper armature c. 

 By inductive action, negative electricity is expelled from B into gg, and 

 the positive held by attraction until the revolving plate, having passed 

 from the influence of c, the -j- electricity is set free, and is gathered up 

 by the points in v v. The negative electricity received by gg passes 

 through n n and /, which we have supposed to be touching ;/, into z, 

 where by induction it charges the armature d with -f. electricity. This 

 reacts on the revolving plate, in consequence of which electricity is 

 collected by //. If after the plate has been turned a few seconds the 

 knobs/ and n are gradually separated, a stream of sparks will continue 

 to pass between the knobs as long as the rotation is continued. A 

 simpler machine subsequently invented by Holtz dispenses with both 

 armatures and openings, retaining only the combs. The interest of 

 these machines consists in the production of electricity by mere motion. 

 There is no friction of surfaces, no chemical change ; but the motion 

 is opposed by the force of attraction between the -f- and electricities, 

 and therefore the electricity is gained only at the cost of the muscular 

 force required to overcome the opposing attraction. 



