SCIENCE AND PRACTICE. oOl 



which would be obtained were no shunt used and the range 

 of the instrument wider. 



47. Charge. This is a method which has two objects : 

 the first is to ascertain the inductive capacity of the cable, 

 and to obtain data for judging of the concentricity of the 

 conductor in the insulating medium ; the second is by observ- 

 ing the loss of static charge by recombination of the elec- 

 tricities through the dielectric, in a given time, to conclude 

 upon the degree of its insulation. 



The connections of the board for measuring charge are 

 precisely the same as those for insulation by deflection. 

 When the cables are long, it is requisite to employ the shunt 

 resistance r r in order to keep the needle within readable 

 bounds. The plug y g is left out when the throw of the 

 needle is to be observed, and the key pressed down. At the 

 moment of completing the circuit the electro-static charge 

 passes through the galvanometer and enters the cable ; the 

 needle is impelled from its position of rest with a sudden jerk 

 to one side, and afterwards continues to oscillate over the 

 zero point until it comes again to rest. The first throw or 

 swing is noted. If the discharge is also to be observed, in 

 order to compare it with the charge, before letting go the 

 key the operator removes the plug from between L' and L, by 

 which the cable end is insulated from the board. 



48. Discharge. As the coils of the galvanometer are 

 seldom entirely free from magnetism, and the needle seldom 

 so exactly centered that the same strength of current gives 

 an equal deflection on each side of the zero line, it is pre- 

 ferable to obtain the first swing of the needle due to the 

 discharge current on the same side as that which was observed 

 for charge. To this end it is necessary only to alter the 

 position of the contact plugs of the galvanometer commu- 

 tator u 2 before the discharge current passes through, by which 

 the galvanometer coils are reversed in relation to the points 

 a and b ; and, as the discharge is in the contrary direction to 

 the charge, the deflection of the needle will be, for both, on 

 the same side of zero. 



After the cable has been left insulated at both ends during 



