14 



Electrical Machine Produces Heavy Charges. 



Thus far we have produced only very small charges, and at the expenditure 

 of considerable labour in producing the friction. However, forty years after 

 Dr. Gilbert's discovery, an electrical machine was invented by which large charges 

 could be generated. At first condensers were not used on the electrical machines, 

 nut shortly afterwards they were applied in the form of Leyden jars. A Leyden 

 jar is an ordinary glass vessel like a fruit jar, covered with tinfoil on the outside, 

 about half way up, and on the inside about half way up. These two tin foils form 

 the two plates of the condenser, and by means of Leyden jars very strong charges 

 can be stored up. 



Figure 15 shows an electrical machine consisting of two glass plates, one of 

 which il rotating and the other stationary. Without the Leyden jars it produces a 

 very fine spark which will jump about two inches of space. When the two Leyden 



Fig. 15. — An electrical machine. It is 

 being turned by crank in rear. Note 

 spark between knobs. 



Fig. 16. — Electrical machine with points 

 attached to positive and negative poles. No 

 flash can be produced. 



jars are connected up the spark is very much stronger, and sufficient to give one a 

 severe shock. It will jump five or six inches of space, and the spark looks exactly 

 like a flash of lightning. 



Electricity Leaks Off Points. 



With this machine one can demonstrate another fundamental principle. If 

 two sharp wires are attached, one to the positive side of the machine and the other 

 to the negative, and the points turned towards each other we find that the machine 

 does not produce a spark and cannot be made to produce one, if the electrification 

 takes place gradually. (Fig. 16.) If a lighted candle is held to one of the points 

 it is extinguished when the machine is turned. There is sufficient wind at this 

 point to blow out the candle. (Figs. 17 and 18.) 



There is only one conclusion possible, viz., that the electricity formed by the 

 machine leaks off these sharp points, preventing sparks from occurring. 



