ELECTRICITY. 



37 



called 'the Discharging Rod or Jointed 

 Discharger, may be conveniently em- 

 ployed. It consists of two bent metallic 

 rods, terminated at one end by brass 

 balls, and connected at the other by a 

 joint, which is fixed to the end of a glass 

 handle, and which, acting like a pair of 

 compasses, allows of the balls being 

 separated at different distances. When 

 opened to the proper degree, one of the 

 balls is made to touch the exterior 

 coating, and the other ball is then quickly 

 brought into contact with the knob of 

 the jar, as represented in Jig. 33, or with 

 Fig. 33. 



any part of the system of the interior 

 coatings, and thus a discharge is effected ; 

 while the glass handle secures the person 

 holding it from the effects of the shock. 



035.) If we wish to send the whole 

 charge of electricity through any par- 

 ticular substance which may be the 

 subject of experiment, we must so ar- 

 range the connecting conductors, as 

 that the substance shall form a neces- 

 sary part of the circuit of the electricity, 

 as it is termed. With this view, we 

 must place it between two good con- 

 ductors, one of which is in communica- 

 tion with the outer coating; and the 

 circuit may then be completed by con- 

 necting the other conductor with the 

 inner coating by means of a discharging 

 rod, to one branch of which, if neces- 

 sary, a flexible chain may be added. 



(136.) In order to direct the charge 

 with more certainty and precision, an 

 apparatus, called the Universal Dis- 

 charger, was contrived by Mr. Henley, 

 and is represented in jig. 34. It con- 

 Fig. 34. 



sists of a wooden stand with a socket 

 fixed in its centre, to which may be 

 occasionally adapted a small table T, 

 having a piece of ivory (which is a non- 

 conductor) inlaid on its surface. This 

 table may be raised and kept at the 

 proper height by means of a screw S. 

 Two glass pillars P, P are cemented 

 into the wooden stand. On the top of 

 each of these pillars is fitted a brass 

 cap, having a ring R attached to it, and 

 containing a joint, moving both verti- 

 cally and horizontally, and carrying on 

 its upper part a spring tube, admitting 

 a brass rod to slide through it. Each 

 of these rods is terminated, at one end, 

 either by a ball, a point, or a pair of 

 forceps, and is furnished at the other 

 extremity with a handle of solid glass. 

 The body through which the charge is 

 intended to be sent, is placed on the 

 table, and the sliding rods, which are 

 moveable in every direction, are then, by 

 means of their insulating handles, 

 brought in contact with the opposite 

 sides, and one of the brass caps being 

 first connected with the outside of the 

 jar or battery, the other may be brought 

 in communication with the inner coat- 

 ings, by means of the discharging rod 

 above described. For some experiments 

 it is more convenient to fix the sub- 

 stance, on which the experiment is to 

 be made, in a mahogany frame, con- 

 sisting of two boards, which can be 

 pressed together by screws, and which 

 may then be substituted for the table 

 T. In either of these ways the charge 

 can be directed through any part of the 

 substance with the greatest accuracy. 



(137.) The quantities of electricity 

 which can be accumulated in any given 

 extent of coated glass, are in the inverse 

 proportion to the thickness of the glass. 

 Different jars or batteries, therefore, 

 will, according to the thinness of their 

 sides, and the quantity of coated surface 

 they contain, have different capacities 

 of holding charges of electricity. But 

 in any given instrument of this kind, 

 the quantity of the charge communicated 

 to it by a machine may be measured by 

 the intensity of the electricity in the 

 prime conductor, which communicates 

 with the interior coating. Some esti- 

 mate of the intensity may be obtained 

 by the employment of Henley's quadrant 

 electrometer already described, ($ 72,) 

 the index of which rises very slowly 

 while the battery is charging, till it 

 reaches a certain elevation, correspond- 

 ing to the capacity of the battery. If 



