Page:Lectures on Ten British Physicists of the Nineteenth Century.djvu/66

 of transmission; the duration of the signal was not distinguished from the time required to traverse the cables. Thomson investigated the phenomenon, and found that it was due to the capacity of the cable; and he deduced the practical result that with cables of equal lateral dimensions the retardations are proportional to the squares of the lengths. This law became known generally as the "law of squares." A Mr. Whitehouse, experimenting with a cable 1125 miles in length, found that the maximum effect of a signal communicated instantaneously at one end was received at the farther end in one second and a half. Applied to these data the "law of squares" said that as the distance from Ireland to Newfoundland is twice the length of the experimental cable, the time in which a signal communicated instantaneously would be received at the further end is 2.52.seconds: that is, six seconds. It became evident that if only five signals could be sent in a minute, the financial success of an Atlantic cable was very doubtful, so Whitehouse fought manfully against the "law of squares." He said, "I can only regard it as a fiction of the schools, a forced and violent adaptation of a principle in physics, good and true under other circumstances but misapplied here." He also made experiments and published results which seemed entirely opposed to the law. To this Prof. Thomson replied in the Atheneum newspaper (Nov. 1, 1856), reiterating the application of the "law of squares" to submarine telegraphy, and showing that the experiments cited really confirmed the law they were supposed to disprove. He further maintained that, notwithstanding the law of squares, Atlantic telegraphy was possible, and stated his conviction that increase of the electric pressure was a development in the wrong direction. Prof. Thomson showed that the condition for rapid signaling consisted in being able to observe the first beginning of the electric current at the far end, and to stop the signal as soon as it had risen to this observable value. To realize these conditions he invented the delicate reflecting galvanometer in which the minute turning of the magnet is magnified by the motion of a spot of light. Maxwell wrote a parody on