Page:Popular Science Monthly Volume 44.djvu/60

 conception is, it has been found by no means an easy one to realize in practice. The first one to attempt the application of this principle to autographic transmission appears to have been Mr. E. A. Cowper. In his apparatus, constructed in 1874, the receiving pen was mounted upon a light armature located between the poles of two electro-magnets placed at right angles to each other. These magnets were included in separate line circuits, and when energized by currents from the transmitting end of the line, attracted the pen armature, causing it to describe a line or curve which was at every instant the resultant of the two right-angled magnetic attractions.

These magnetic attractions were varied in exact accordance with the movements of the transmitting pen by augmenting and diminishing the strength of the current flowing through the magnet coils, and this variation of current strength was in turn accomplished by causing contacts attached to the transmitting pen to pass over the terminals of resistance coils and successively cut out or introduce resistance in the line circuits. On account of the very limited movements which could be given to both the transmitting and receiving pens, the writing had to be done upon a strip of paper which was moved under the pen. The writing with the transmitting pen was done through a square hole about an inch on a side, and the characters had therefore to be made practically one over the other. There was thus but little opportunity for the operator to follow the work and see clearly what he was doing, and only an expert could make an intelligible writing. The details of this method were subsequently much improved by two American inventors, and the apparatus employed for a time in commercial work; but the essential limitations of the method proved too serious a handicap, and the system soon fell into disuse.

In taking the subject up experimentally Prof. Gray at first used the method of a variable resistance, but he speedily abandoned it as impracticable, and adopted the step-by-step method of operation, which he now uses. This consists in causing the transmitting pen to send to the line a succession of distinct electrical impulses, the number of which is governed by the extent of the pen's movement, which are employed, not in affecting the