48 ELEMENTARY LESSONS ON [CHAP. i. 



and having the pointed ends bent round at right 

 angles. When placed on the prime conductor of the 

 machine, or joined to it by a chain, the force of 

 repulsion between the electricity of the points and . that 

 on the air immediately in front of them drives the 

 mill round in the direction opposite to that in which the 

 points are bent. 



Another favourite way of exhibiting electric repulsion 

 is by means of a doll with long hair placed on the 

 machine ; the individual hairs stand on end when the 

 machine is worked, being repelled from the head, and from 

 one another. A paper tassel will behave similarly if 

 hung to the prime conductor. The most striking way 

 of showing this phenomenon is to place a person upon 

 a glass-legged stool, making him touch the knob of 

 the machine ; when the machine is worked, his hair, 

 if dry, will stand on end. Sparks will pass freely 

 between a person thus electrified and one standing 

 upon the ground. 



The sparks from the machine may be made to kindle 

 spirits of wine or ether, placed in a metallic spoon, 

 connected by a wire, with the nearest metallic conductor 

 that runs into the ground. A gas jet may be lit by 

 passing a spark to the burner from the finger of the per- 

 son standing, as just described, upon an insulating stool. 



44. Armstrong's Hydro-Electrical Machine. 

 The friction of a jet of steam issuing from a boiler, 

 through a wooden nozzle, generates electricity. In 

 reality it is the particles of condensed water in the jet 

 which are directly concerned. Sir W. Armstrong, who 

 investigated this source of electricity, constructed a 

 powerful apparatus, known as the hydro-electrical 

 machine (Fig. 28), capable of producing enormous 

 quantities of electricity, and yielding sparks five or six 

 feet long. The collector consisted of a row of spikes, 

 placed in the path of the steam jets issuing from the 

 nozzles, and was supported, together with a brass ball 



