334 Dynamic Theory. 



around the great magnets, and thus supply them with additional force, 

 which being done the current sent off from the bobbins at once greatly 

 : ncreased. Some of the dynamos now in use adopt this plan, a part of 

 the current generated in the revolving bobbins being switched off or 

 "shunted " from the main current and made to run around the electro- 

 magnets and give them greater force, which greater force reciprocally 

 acts to increase the main current. The part of the main current not 

 returned to reinforce the electro-magnet passes on to do the work of 

 electro lighting, &c. 



Two conductors separated by an insulating body in a thin stratum, is 

 an Electric Accumulator. A Leyden jar, is an example. The two con- 

 ductors are the electrodes, and the insulator is called a dielectric. Thus 

 in the Leyden jar, the layers of tin foil are the electrodes, the glass is 

 the dielectric. 



Conductors. The passage of electricity from its potential to a neutral 

 position is much more rapid in some than in other bodies; the time 

 elapsing after the discharge before the potential is uniform over the 

 whole body, being very different in different bodies; this difference in- 

 dicating difference of resistance. All metals are good conductors, but 

 not alike. The resistance of lead is 12 times that of copper or silver ; 

 iron is 6 times that of copper or silver ; mercury is 60 times that of 

 copper or silver. The higher the temperature of metals the greater is 

 the resistance. All liquids containing water, and all damp bodies, are 

 conductors, inferior to metals, and cannot be used for insulators. Gases 

 at the atmospheric pressure, whether dry or moist, are insulators so 

 nearly perfect, when the electric tension is small, that there is no evi- 

 dence, as yet, of electricity passing through them by ordinary conduc- 

 tion. When electricity passes through gases, it is by disruptive dis- 

 charge on account of high tension. The electric strength of a dielectric 

 or insulator, is the value of the electromotive force which can exist in 

 a dielectric without causing a discharge. The electric strength of air 

 diminishes as the pressure is reduced from the atmospheric pressure to 

 that of about three millimetres of mercury. ' When the pressure is 

 still further reduced, the electric strength rapidly increases, and when 

 the exhaustion is carried to the highest degree hitherto attained, the 

 electromotive force required to produce a spark of a quarter of an inch, 

 is greater than that which will give a spark of eight inches in air at or- 

 dinary pressure.' (Clerk Maxwell.) The most perfect vacuum yet 

 formed is an insulator of very great electric strength. The electric 

 strength of hydrogen is much less than that of air. Cold glass is a 

 good insulator, but when hot, say 200 F. , is a conductor. Gutta- 

 percha, caoutchouc, vulcanite, paraffin and resins are good insulators, 

 the resistance of gutta percha at 75 F., being about 6 X 10 19 (ten 19th 

 \ 



