38 



ELECTRICITY. 



the electricity be accumulated beyond 

 this limit, a spontaneous discharge takes 

 place, and the process must then be re- 

 newed in order to obtain a full charge. 

 It is more prudent, however, to stop 

 before this degree of accumulation is 

 attained: and one great advantage of 

 Henley's electrometer is, that it shows 

 us the progress of the charge, and how 

 far we may proceed with safety. 



(138.) But the most effectual security 

 against fracture from a spontaneous 

 discharge, is to form an interrupted 

 circuit, of which the parts, where the 

 interruption occurs, terminate by me- 

 tallic balls, placed at a certain distance 

 from each other. By varying the in- 

 terval between them, we may regulate 

 the quantity of electricity which we 

 shall allow to accumulate in the battery ; 

 for the moment it exceeds the quantity 

 of which that interval is the striking 

 distance ( 87,) an explosion happens, 

 by the electricity forcing its w r ay through 

 the air from one ball to the other. If the 

 balls be brought very near each other, 

 a discharge will take place with a com- 

 paratively small accumulation: when 

 farther separated, a greater charge will 

 be retained, because a higher intensity 

 of electricity is required in order to pass 

 through the larger intervening space. 

 It is on this principle that the instru- 

 ment, called Lane's Discharging Elec- 

 trometer, is constructed. It consists of 

 a brass ball, ~B,fig. 35, placed at the end 



Fig. 35. 



quired : the other end of the moveable 

 rod must be connected, by means of a 

 chain or wire, with the outer coating. 



The chief use of this instrument is to 

 allow a jar to discharge itself sponta- 

 neously through any previously arranged 

 circuit, without employing a discharging 

 rod, or moving any part of the apparatus ; 

 and also to produce successive explo- 

 sions nearly of the same strength. The 

 magnitude of the charge is measured by 

 the distance at which the balls are 

 placed ; and the power of the machine 

 may be estimated by the number of 

 explosions, which, at any given dis- 

 tance, take place in equal times. In 

 Mr. Lane's experiment the shocks were 

 twice as frequent when the interval be- 

 tween the balls was l-24th of an 

 inch, as when twice as much : hence he 

 concluded that the quantity of electri- 

 city required for a discharge is in exact 

 proportion to the distance between the 

 surfaces of the balls. But the indica- 

 tions of this instrument are in reality 

 subject to great fallacy, on account of 

 the variable state of the atmosphere, 

 which affects its conducting power; 

 the quantity of dust which, even during 

 the course of an experiment, is liable 

 to be attracted, and to collect upon the 

 balls ; and also from the roughening 

 and tarnishing of the metallic surfaces 

 produced by frequent electric explo- 

 sions. This last imperfection is one to 

 which brass balls are particularly ex- 

 posed ; and might, if it were worth 

 while, be remedied by having the balls 

 made of fine silver. 



(139.) Another contrivance for re- 

 gulating the amount of the charge 

 which we may wish to send through any 

 substance, is that invented by Cuth- 

 bertson, and termed the Balance Elec- 

 trometer. It consists of a metallic rod, 

 R, fig. 36, terminated by two equal 

 balls A, B, and balanced, like a scale- 



Ftg. 36. 



of a short metallic rod R, which moves 

 through a tubular piece, supported by 

 a bent glass stand S. This stand is 

 made so as to be capable of being fixed, 

 by its other extremity, to the rod pass- 

 ing up from the interior coating, and 

 adjusted so that the ball B is immedi- 

 ately opposite to the knob' of the jar, 

 and may be brought to the exact strik- 

 ing distance from it which may be re- 



