370 SUBMAEINE CABLE LAYING ANP EEPAIEING. 



Weston measuring instruments were the first with permanent 

 magnets of the split-ring type which could be relied upon to 

 maintain their magnetisation under all external influences. By 

 the use of steel of a composition having great retentive power 

 and its proper ageing after magnetisation, instruments were pro- 

 duced which could retain their true calibration notwithstanding 

 severe external current and magnetic influences. The Weston 

 milammeters were used in cable testing by the author and others 

 very soon after their appearance on the British market, and 

 were found extremely convenient. Instead of laborious cal- 

 culations to find the current passing through the bridge to line, 

 involving a separate measurement of the battery resistance, the 

 milammeter placed in the line indicated at once the currents 

 or ratios of currents passing and their direction. Deadbeatness, 

 too, was an inestimable advantage, this being attained by wind- 

 ing the coil on a closed frame. The possibility of seeing whether 

 the testing battery current kept up or fell oflf during a test was 

 another great advantage. Any variation in the testing current 

 due to cells polarising or varying in E.M.F. or resistance could 

 be traced and remedied with great facility. 



Previous to the adoption of the milammeter giving a con- 

 tinuous indication of the current during a test, any wildness 

 in the observations was usually attributed to the fault, but 

 was frequently due to variation or polarising of the cells. The 

 milammeter changed all this : the current indications could be 

 watched, and any falling off corrected at once by a slight read- 

 justment of resistance in the battery circuit. 



In a series of very able and useful articles in The Electrician, 

 Vol. XLIX. (1902), pp. 677, 707, 755 and 793, entitled "The 

 Capabilities of the Milammeter and the Galvanometer in general 

 in Submarine Cable Testing," Mr. C. W. Schaefer showed in 

 detail the simplifications effected by the use of this instrument 

 in practically every fault or break test. He pointed out the 

 advantage of the milammeter in break methods where the seal- 

 ing-up of the exposure under a positive current could be watched 

 by its indications. It is very necessary to observe this effect, 

 because if the end is buried the break appears to be beyond its 

 true position. Mr. Schaefer also gives details of design of a 

 milammeter with three or more ranges to indicate from ^V to 

 150 milliamperes, and to be unaffected by temperature variation. 



