Electrical Machines. 255 



I have already observed that any conducting surface in connexion 

 with either conductor, must act as a part of that conductor. Ap- 

 proximating to the negative conductor, a body (a ball, for instance,) 

 while in communication with the positive conductor, is really enlarg- 

 ing on elongating the surface of the latter, so that when the spark 

 passes, it must still be from the positive to the negative pole: and 

 vice versa, elongating the surfaces associated with the negative con- 

 ductor, till sufficiently near the positive conductor to receive a spark, 

 does not alter the character of the phenomenon. In each case, ac- 

 cording to the theory of one fluid, a current passes from the positive 

 to the negative pole, and according to the doctrine of two fluids, two 

 currents pass each other. 



The cause of the difference observed in the sparks in the two cases 

 is, that they are usually received from a small knob upon a big ball, 

 or the hand ; or some other body comparatively large. 



Whenever the fluid is contracted into a small jet on the positive 

 side, its projectile power is increased ; while under the opposite cir- 

 cumstances, its projectile force is lessened. This is the sole cause 

 of the long forked erratic form, of what is called the positive spark; 

 and the short stubbed appearance of what is called the negative 

 spark. The whole difference may be effected in whatever situation 

 the sparks may be taken, by causing a large and a small ball to ex- 

 change sides. When the surface on the positive side is so small as 

 to condense the electric matter before it jumps, the projectile force is 

 greater, and as in the case of the jet pipe in hydraulics, there is a me- 

 dium size, at which the greatest projectile power is obtained. When 

 the emitting surface is too large, the projectile force is lessened, and 

 the spark consequently made shorter. 



The following passage in Cavallo's Electricity is that alluded to 

 above. See vol. 1st, page 184: London, 1786. 



** Sometimes the machine will not work well because the rubber is not sufficient- 

 ly supplied with electric fluid ; which happens when the table upon which the ma- 

 chine stands, and with which the chain of the rubber is connected, is very dry, and 

 consequently in a bad conducting state. Even the floor and the walls of the room 

 are ia very dry weather bad conductors, and they cannot supply the rubber suffi- 

 ciently. In this case the best expedient is to connect the chain of the rubber, by 

 means of a long wire, with some moist ground, a piece of water, or with the iron 

 work of the water pump, by which means the rubber will be supplied with as much 

 electric fluid as is required." 



The learned author was, I think, altogether wrong in imagining that 

 the dryness of adjacent bodies could have any ill effect. In common 



