ELECTRIC MACHINE 



1993 



ELECTRIC MACHINE 



the small charges have been added together 

 to produce very strong charges on the knobs 

 of the machine. This principle, namely, the 

 accumulation of small charges to produce a 

 strong charge, is true of all electric machines, 

 whether the electrification is produced by fric- 

 tion or by induction. 



The induction electric machine is more ef- 

 fective than the friction machine. The former 

 is used to-day in physics laboratories and in 



LEYDEN JARS 



X-ray work; it is a device for charging with 

 opposite charges two knobs connected with 

 the inner coatings of two Leyden jars (see 

 LEYDEN JAB). The outer coatings of the jars 

 are connected together, the outer coating of 

 each jar having a charge of the kind opposite 

 to that of its inner coating. The distribution 

 of the charges is shown in the illustration. 

 Electric charges are continuously added to the 

 knobs, the charges on the outer coatings of the 

 jars being correspondingly increased. When 

 the attraction between the two charges equals 

 the breakdown strength of the air a spark 

 passes between the knobs. If the knobs are 

 connected to an X-ray or a Geissler tube there 

 is a continuous discharge through the tube. 



The Common Machine.- The knobs a and b 

 are connected to two collecting combs, as 

 shown in the second illustration. These combs 

 collect electric charges from metal studs on 

 a rotary glass or vulcanite disk. These studs 

 are charged by induction at the instant they 

 touch the brushes of the rod c. Charges are 

 induced on rod c by the charges on the sheets 

 of tinfoil and varnished paper on the back 

 of the fixed disk. The studs give up part of 

 their charges to the sheets of tinfoil, so the 

 charges on the tinfoil as well as on the knobs 

 are increased by the rotation of the disk. 



The energy of the electric charges is pro- 

 duced by the work done in rotating the disk. 

 It is not correct to say that electricity is pro- 

 duced by the action of the machine. There 

 is as much electricity in an uncharged metal 



rod as in the same rod with equal positive and 

 negative charges at its ends; the machine 

 merely produces a tension or strain in the 

 electricity which already existed. A body in 

 which this electrical force or strain exists m 

 electrically charged. 



Experiments with the Machine. If a person 

 stands on a stool the legs of which rest on 

 glass or rubber and touches one knob of the 

 machine while it is being turned, his body 

 becomes electrically charged. His hair, if dry. 

 will stand on end. If another person Jiolds 

 his hand near the hair of the person who ia 

 receiving the charge the hair will turn toward 

 his hand, because of electrical attraction. The 

 person on the stool has no unpleasant sensa- 

 tion, no matter how strong the charge may be, 

 for there is no sensation until a discharge takes 

 place. If the person on the stool touches the 

 hand of another person a spark passes be- 

 tween their hands, and each experiences a 

 sharp pricking sensation. A number of per- 

 sons, holding hands, may form a line; let the 

 machine be turned until a small charge col- 

 lects on the knobs and then the two at the 



COMMON FORM OF ELECTRIC MACHINE 



ends of the line may touch the two knobs aft 

 the same instant. Every person in the line 

 will feel the discharge, but it will not be in- 

 tense enough to be painful. 



An experiment which is beautiful, especially 

 in a darkened room, may be performed as 

 follows: Drawa design in outline on paper. 

 Then lay a plate of glass on the paper and 

 paste bits of tinfoil or tea lead on the glass, 

 following the outline on the paper beneath 

 and leaving spaces of about one-eighth inch 

 between the bits of tinfoil. Then attach two 

 wires to the knobs of the electric machine and 

 let the ends of the wires touch the pieces of 

 tinfoil at the ends of the design. When the 

 machine is turned sparks pass across all the 

 spaces between the bits of tinfoil, illuminating 

 the design. 



