1862.] 213 



were connected with two wires of a galvanometer, and one half of 

 the wheel was connected with the receiving end of the cable wire 

 tested, and the other half of the commutator was connected with 

 the earth. 



The two commutators were so arrarfged that when, by the rotation 

 of the wheel, the current of electricity from the battery was reversed, 

 the connexions of the galvanometer were reversed also ; and there- 

 fore, if the speed of the electric wave through the cable were inde- 

 finitely great, the currents would flow through the galvanometer in 

 one direction, no matter how fast the currents in the cable are re- 

 versed. As, however, a given amount of time elapses before these 

 waves reach their maximum at the distant end of the circuit, and 

 as also a given time elapses after the battery has been reversed 

 at the one end before the current is reversed at the other or 

 distant end, it is clear that by gradually augmenting the rate of 

 rotation of the commutator until the wheel is a quarter of a revo- 

 lution in advance of the wave, a point is arrived at when the galva- 

 nometer's connexions are reversed precisely at the moment that the 

 wave reaches its maximum strength, and consequently the wave is bi- 

 sected, one half of it flowing through the galvanometer in one direction, 

 and the other half in the other. At this rate of rotation the galva- 

 nometer falls to zero ; because, the wave being exactly bisected, the 

 one half tends to deflect the needle to the right, and the other to the 

 left, but, owing to the weight of the needle and the rapidity of the 

 reversals, it (the needle) stands nearly steadily at zero. The galva- 

 nometer used consisted of a rather heavy astatic pair of needles 

 suspended by a silk fibre. The wire acting upon the needles was 

 about the twentieth part of an inch in thickness, in order that it 

 should offer no serious resistance to the electric current. Its re- 

 sistance was less than one Varley unit (1 mile copper wire T x inch in 

 diameter). 



The rate of rotation necessary to obtain the first zero is the point 

 recommended for comparing the relative speeds of the electric waves 

 through submarine cables of different dimensions. 



By augmenting the speed beyond that necessary to produce the 

 first zero, the needle becomes deflected in the opposite direction and 

 gradually approaches a maximum ; that is to say, when the electric 

 wave is half a revolution behind, the currents all flow through the 

 galvanometer in one direction again. This is termed the second 



VOL. xii. a 



