4O THE ADDRESSES, LECTURES, ETC., OF 



This is one method of determining the charge, or rather the 

 capacity, of a cable. 



Another method is by the employment of an instrument which 

 we call the " wippe" It being very difficult to read the sudden 

 deflection of a needle when connection is made with the battery, 

 we substitute by this improved method the constant deflection of 

 that needle. The instrument by which we accomplish this object, 

 and which is placed upon the table, consists of an electrical appli- 

 ance for producing uniform rotation of a spindle, by means of 

 which connection is made, at rapid intervals, alternately between 

 the cable to be measured and the battery, on the one hand, and 

 the earth on the other hand, producing a regulated succession of 

 charges and discharges. Suppose I make one hundred, or say 

 twenty, charges in a second, and twenty discharges. All these 

 charges go through my instrument in rapid succession, and the 

 needle has not time between impulse and impulse to return to 

 zero, but it will subside into a medium position, which we can 

 examine at our leisure. The charge in this case is not proportionate 

 to the sine of half the angle, but to the sine of the angle of 

 deflection itself. In like manner the instrument enables us to 

 measure the discharges of the cable to earth, allowing the charges 

 to go into the cable direct. We have thus a ready means of care- 

 fully examining the capacity of any given cable. Though I could 

 put this instrument now to work, it could only be seen by two or 

 three gentlemen, and even those would require more leisure than 

 we could afford them at present, in order to satisfy themselves of 

 the result ; these are, indeed, not experiments for a lecture-room, 

 but for the study. 



OKDER OF TESTS. I think I have passed through all the 

 principal modes of measuring resistance and charge. I should say 

 a few words now about their application in practice. We have to 

 test the cable in all its stages of progress, in order to guard against 

 the possibility of a fault. Commencing at the insulating works, 

 or say at the gutta-percha works, we have first to examine the wire 

 which is proposed to be covered. As I have shown at the begin- 

 ning of the lecture, the conductivity of copper wire varies between 

 wide limits ; and unless great care is used, unless every portion of 

 the wire is carefully examined, we might lose enormously in con- 

 ductivity. It will be seen at once of what importance conductivity 



