THE LOCALISATION OF BREAKS AND FAULTS, 537 



tion. The curve would also show at a glance whether the 

 general course of the observations was right. 



Curve A, Fig. 303, shows the relation between various testing 

 currents observed on & milammeter and the corresponding 

 bridge readings to false zero in a break test. Now, by using 

 the inverse square root of the currents according to Kennelly's 

 law, Mr. Jona obtained an approximately straight line C as 

 one would expect when the ratio of two variables is a constant. 

 It is nob always an absolutely straight line in practice, because 

 of errors in testing, and the fact that the law is only strictly 

 correct) when the exposure is perfectly clean, but the straight 

 line gives at once an average and yields the same results as 

 the calculations. He showed that this curve or line has the 

 useful property that, if produced dov/n wards, it cuts the base or 

 resistance line at the point corresponding to the break. 



The extension of these curves downwards is theoretically 

 equivalent to increasing the current and lowering the resistance 

 of the fault until at the base line the current is infinite and the 

 fault or break resistance nil. This is purely a mathematical 

 conception, but one which, as will be seen can be usefully applied. 



In Fig. 304 curve B shows bridge readings on the same break 

 to true or scale zero in relation to the currents employed, and 

 curve D the same readings plotted in relation to the inverse 

 square root of the currents. It will be seen that if curve D is 

 produced in its natural direction downwards, as shown by the 

 dotted line, ic cuts the resistance line as before at a point 

 giving the resistance up to the break. The distance of the 

 break indicated by the curve is between 507 and 508 ohms, 

 and the actual distance in this case was 506 ohms. 



This useful property of the curve is turned to good account 

 by plotting observations and extending the curves as 

 described, when a very close approximation to the break 

 position is obtained by inspection, and it can also be seen 

 more clearly and quickly than by a set of figures alone which 

 observations are uniform and which should be rejected, and 

 whether the right currents have been used. Several cable 

 ships now keep records of break or other localisation curves for 

 reference, and for this purpose have adopted Mr. Raymond- 

 Barker's calculator board (Fig. 305) as a convenient means for 

 plotting them. This device is a board about 21 in. square, 



