38 THE ADDRESSES, LECTURES, ETC., OF 



the latter. For instance, if I have in the galvanometer circuit a 

 resistance of one million units, I know that in order to balance 

 the needle on the galvanometer, I must screw the moveable coil to 

 a certain point on the scale ; then, by increasing that resistance, I 

 must diminish the relative effect of the moveable coil, in order to 

 maintain the needle in balance by moving to a point which can be 

 fixed beforehand by calculation, or which may easily be tabulated 

 from a series of observations in dealing with known resistances. 

 In working with this instrument the unknown resistance is put 

 into the galvanometer circuit, and the screw of the moveable 

 resistance is worked to and fro, until the needle has assumed its 

 zero position, when in reading upon the scale the divisions, I find 

 at once in my table the amount of resistance in units which I wish 

 to measure. Such an instrument is particularly useful where you 

 want to operate quickly, as you do on board ship. In paying out 

 cable, it is of the utmost importance that, when a fault goes over- 

 board, you should at once be able to determine the magnitude of 

 that fault ; and this can be accomplished in much shorter time 

 with this instrument than with the instruments we have formerly 

 employed. 



DISCHARGE METHOD. Another method of measuring a very 

 high resistance is by natural discharge. If we have only a short 

 and highly insulated wire, and charge the same with electricity, 

 the external surface being connected to earth, we shall find that a 

 natural discharge takes place in time, which latter varies according 

 to the insulating property of the covering. This diminution will 

 go on at a very uniform rate. If I employ a very high electro- 

 motive force, say one hundred cells, I produce a high state of 

 tension ; this tension produces a comparatively rapid discharge, 

 and at the end of a given time say in one minute, it will have 

 descended to half that charge. If, on the other hand, we employ 

 a weak current or electro-motive force, we shall have only a small 

 quantity of electricity to discharge, and it will be found that this 

 charge will be reduced to one-half in precisely the same time. In 

 employing this method we are perfectly independent of the amount 

 of battery power employed, but in taking the charge we must note 

 the time ; in connecting now the wire with an electrometer, we 

 shall have to watch this electrometer in order to note when this 

 charge has dropped down to half charge. The time of discharge 



