SUBMARINE TELEGRAPHY 251 



same arbitrary unit. To obtain further subdivision, a second set 

 of resistance coils, each equal to one-fifth of each of those of the first 

 slide, are joined so as to bridge over any two of the first set. The 

 first set consisting of eleven equal coils, the total resistance between 

 A and B will always be ten units, and the ten studs of slide 2 will 

 subdivide into ten equal parts the potential between the two studs 

 of slide 1 which they bridge. Thus, as drawn, calling the total po- 

 tential of A zero, and of B 100, the potential of X will be 35. Slides 

 1 and '2 during the test are moved till the potential is found, which 

 is exactly equal to that of the cable. In reality, there were 101 

 equal coils in slide 1, and 100 in the second slide, each equal to -^th 

 part of those in the first. Thus the potential between A and B 

 could be subdivided into 10,000 equal parts. 



Explanation of Diagram of Tests on board Ship. 

 The diagram, when connection is made by plugs at P 2 and P,, 

 but broken at P, represents a Wheatstone's balance, the four arms 

 of which are. 1st, a set of resistance coils No. 1 between Z and AV ; 

 _nd, part of the slide resistance coils between X and Z ; 3rd, the 

 remainder of the slide resistance coils between X and Y ; and 4th, 

 the gutta-percha sheath of the cable between the conductor, Yj, and 

 the earth, Y. G is the galvanometer of the balance, arranged with 

 shunts to diminish its sensitiveness when required. The resistance 



XY 



of the gutta-percha = resistance coils No. 1 multiplied by _ . Thus 



-XZ 



if the slide reading had the value 67, as shown in the diagram, we should 

 have gutta-percha resistance = resistance coils No. l-f-O^j? !) 



In the actual slides with 100 coils each, the resistance in coils 

 No. 1 was divided by 10,000, divided by the slide reading, and a 

 unit subtracted. The same formula gives the copper resistance if 

 the remote end of the copper conductor is put to earth. Resistance 

 coils No. 2, and galvanometer G,, were used to maintain a constant 

 current through the slides as described for the shore tests. Y was 

 usually connected to earth by the bar C which joins and b, but 

 when the keys a or 6 were depressed, they sent signals to the shore 

 from battery No. 2, but these signals only slightly raised or lowered 

 the potential of the cable, and did not much disturb the electrifica- 

 tion. When signals were being sent, the sensibility of the galvano- 

 meter G was diminished by shunts, which were removed during the 

 pause after each word to take a perfect insulation test. If anything 

 had gone wrong with the "Wheatstone balance arrangement, the plugs 

 P., and Pj would have been withdrawn, and connection made at P, 

 which would have established the old direct simple insulation test 

 from C to Z, P, G, W, and Y,. 



