6 ELECTRICAL MEASUREMENTS 



ance of the cable and demonstrated that, after the key at tin- 

 sending end is depressed, a definite time, a, must elaps. I 

 current is received at the far end, that the received current thru 

 gradually rises, attaining 90 per cent, of its full value only after t hr 

 lapse of a time equal to 10a and that when the key is released the 

 received current gradually falls to zero. He also showed that t hr 

 time which must elapse before any signal is received depends 

 upon the square of the length of the cable. These were weighty 

 matters, for the possibility of a transatlantic cable was then under 

 discussion. Having demonstrated the shape of the "arrival" 

 curve, Professor Thomson invented his galvanometer as a form 

 of receiving instrument adapted to cope with these difficulties 

 and render an Atlantic cable an economic success by increasing 

 the speed of signalling. 1 * 



What he desired was an instrument which would be deflected 

 by minute currents and follow their every fluctuation. The 

 original instrument used on the frigate Niagara in 1858 is shown 

 in Fig. 4. 



The essential features of the Thomson galvanometer are: 



1. A very small and light movable magnetic system delicately 

 suspended within a coil so proportioned that its turns are in 

 close proximity to the needle. 



2. A small needle, controlled by the combined action of the 

 local magnetic field and the field due to a permanent magnet. 



3. The magnification of the motion of the needle by the use 

 of a beam of light as an index. (A small concave mirror is 

 attached to the needle and throws an image of the filament of 

 an incandescent lamp on a graduated scale from which the de- 

 flections are read. The equivalent of a very long pointer with 

 a small moment of inertia is thus obtained. If a plane mirror 

 is used, the reading is effected by a telescope and scale.). 



In the older forms of the instrument a simple magnetic needle 

 was used. This was cemented to the back of the mirror which 

 was suspended by a very short silk fiber in a tube of diameter 

 slightly greater than that of the mirror. The tube was closed 

 at one end by a piece of plate glass and at the other by a movable 

 plug. As the suspended system swung in a constricted space 

 its motions were damped by air friction. 



* Numbers refer to references at end of chapter. 



