But of course there is a severe limitation to the usefulness of this machine, inasmuch as the drawing has to be made at the time of transmission, and it can only be "put on the wire" by the hand of the artist himself.
CHAPTER XIV
A WONDERFUL EXAMPLE OF SCIENCE AND SKILL
In the preceding chapter reference was made to the fact that for the successful sending of pictures "by wire" one thing was necessary above all others. That one thing consists in making two machines, perhaps hundreds of miles apart, start working together, stop together and, when working, turn at exactly the same speed. Let the reader just picture the problem to himself, and ask himself how such an arrangement can be possible. Let him think of a town two hundred miles away and then meditate on the possibility of making a machine working in his own room and another in that distant town maintain perfect unanimity in their movements. The result of such reflection will probably be the assertion that such a thing is beyond the bounds of possibility. Then he will find the following description of how it is done extremely interesting.
In the first place it must be understood that each machine is driven by an electric motor. The motors are designed to run at 3000 revolutions per minute, and they drive the cylinders of the machines through gearing so arranged that the latter turn at 50 revolutions per minute.
Now of all machines perhaps the most docile and easily managed is the direct-current electric motor. Each such machine is made with a view to its working at a certain speed, but that can be varied within certain limits, by simply varying the force of the current which drives it. And that force can be very easily varied by the use of an instrument called a "rheostat" or variable resistance. We are all familiar with the way in which the engine-driver regulates the speed of a locomotive, by means of a valve in the steam-pipe. The opening and closing, more or less, of the valve enables the speed to be changed at will and adjusted to a nicety. The rheostat is to the electric current what the valve is to the steam; it can be opened and closed, more or less, as necessary. By it the current driving the motor can be made stronger or weaker, and as that change is made so does the speed of the motor change accordingly. Thus we see that there is at hand the means of setting a motor to work at any desired speed.
The difficulty, however, is to tell when the desired speed has been attained. One can count the revolutions of a machine at two or three revolutions per minute with a certain amount of accuracy, but fifty revolutions per minute are more than one could count correctly. Still less could we count the 3000 revolutions every minute of the motors. Thus, even if we had the two motors side by side, we should have extreme difficulty in making them work at the same speed exactly. One might be doing 3000 while the other did 2990 or 3010 and we should be none the wiser. And when we separate the two by a distance of many miles, the task of synchronising them is even worse.
But fortunately there is a simple contrivance by which we can tell very accurately the speed of a motor. The reader has already been familiarised, in previous chapters, with the difference between direct or continuous electric currents and alternating ones. It is the continuous sort which is used to drive these motors, but a slight addition to the machine will make it so that while direct current is put in, to drive it, alternating current can be drawn out of it. Two little insulated metal rings are fitted on to the spindle of the machine, and these are connected in certain ways to the wires of the motor; then against these rings, as they turn, there rub two little metal arms, called, because of their sweeping action, brushes; and from these brushes we can draw the alternating current.