372 SUBMAEINE CABLE LAYING AND KEPAIKING. 



according to the range required. The coil is balanced and turns 

 in jewelled pivots, and the directive force is obtained by means 

 of fine spiral springs. The scale is divided into 150 divisions, 

 and on the 150 range each division equals one mllliampere. 

 On the 50 range a current of 50 milliamperes will deflect to 

 150 divisions, and therefore each division equals one-third of a 

 milliampere. The most sensitive, or G, range is the natural 

 calibration of the coil unshunted, which, in the case of the 

 instrument illustrated, is 7'66 milliamperes for 150 divisions — 

 that is, each division equals 0-05105 milliampere. The wire 

 connected to the positive terminal is kept on for all ranges, 

 this being the common return terminal. The other wire is 

 connected to the terminal marked with the range required. 

 The zero is made 10 divisions in advance of one end of the 

 scale, so that the instrument qan be used for speaking by re- 

 versals between ship and shore. 



The resistance of the unshunted coil (G range) is about 

 10 ohms, and in the other ranges the shunted coil is under 

 2 ohms. So that using the 50 or 150 range the resistance of 

 the instrument may generally be neglected when it is connected 

 in the line. When the largest of the two currents used in test- 

 ing is a little over the maximum for the G range (in this case 

 about 8 milliamperes), better indications will be obtained by 

 using the 50 range and connecting the instrument in the battery 

 circuit as in Fig. 248. When this is done the readings must be 

 halved for an even bridge, because only half the battery current 

 goes to line. Also, if the instrument is between the battery and 

 key it only indicates one way for both directions of current. 

 But the advantage is gained of larger deflections, and conse- 

 quently greater accuracy in reading. Supposing, for instance, 

 the currents were 4^ and 10 milliamperes. This could not be 

 taken on the G range, and if the instrument were in the line 

 the deflections would be only 13| and 30 respectively on the 

 50 range. By putting it in the battery circuit with the 50 

 range the readings become 26 and 60 respectively, which are 

 much more easily observed with accuracy. 



An improvement would be to provide a 25 milliampere 

 range, giving readings of 6 divisions per milliampere. Then 

 the instrument could be used in the line with sufficient sensi- 

 tiveness when the larger currents of a pair were from, say, 8 to 



