THE LAYING OF SUBMARINE CABLES. 161 



vanometer as described above (Fig. 75). When used for this 

 purpose the terminals marked Z and C are connected to the 

 galvanometer and the other two terminals respectively to one 

 pole of the battery and the line. With these connections any 

 current that is sent to line passes through the galvanometer in 

 one or other direction, according as the lever is to the right or 

 left, and the galvanometer can be short-circuited at will by 

 putting the lever to the central position and closing the short- 

 circuit key. As regards construction, the contacts are all 

 visible, and the central and other pillars are secured under- 

 neath the base by thumb-screws, so that the key can be taken 

 apart for cleaning without the use of any tool. It Is mounted 

 on the usual "Bavarian" base 6 Jin. by 4Jin., and fixed to the 

 test table by ebonite pillars without screws, in the excellent 

 manner now usually adopted. This method is to mount the base 

 of each instrument on four pillars or feet (each of fin. diameter 

 and Jiu. long) which are made to step into corresponding 

 holes in wooden strips fastened to the table. This is found 

 a perfectly safe fixing for shipboard, and avoids the surface 

 leakage due to damp, &c,, which occurs when the bases touch 

 the table. Cleaning is also more easily done than when the 

 bases are screwed down. 



A small and simple reverser, designed by Mr. Charles Bright, 

 is shown in Fig. 79. The central disc is in two halves 

 insulated from each other, each with two contact points bear- 

 ing on two quadrants. The terminals marked Z and C are 

 connected either to the battery or galvanometer according 

 as the instrument is used as a current or galvanometer 

 reverser. 



The marine reflecting galvanometer devised by Lord Kelvin 

 (Fig. 80) has been a useful instrument in its time, and is 

 still retained in some ships in a modified dead-beat form. 

 Generally speaking it has been superceded by the moving 

 coil form of instrument to which reference is made in 

 Chapter V. The mirror (with the needles cemented at the 

 back) is suspended on a fine silk fibre mounted in the brass 

 frame shown in Fig. 81. The lower end of the fibre is fixed, 

 and the top end passes out through a central hole in the frame, 

 and is attached to the spring S, which puts a slight tension on 

 the fibre. When the mirror is suspended, the frame is held by 



