But even he does not attempt to do it with one wire, it should be said, for he uses two. This is no drawback when, as is often the case, it is used in conjunction with a telephone, for the latter, to be effective, also requires two wires. Years ago single wires were employed for telephones as for telegraphs, the circuit being completed through the earth. But the difficulty arose that every wire through which currents flow is apt to induce currents in neighbouring wires--the induction coil is based upon that fact--and so messages in one wire were overheard on others, or, what was perhaps more annoying still, the dots and dashes passing in a telegraph wire would produce loud noises in a telephone wire that happened to be near. The use of two wires, however, entirely removes that trouble, for the neighbouring current then induces two currents instead of one, one in each, and it so happens that these are opposed to each other, so that they neutralise each other. So every telephone wire now is double and therefore is ready, as it were, to have the telewriter fitted to it. But even with two wires the difficulty seems insuperable until we remember that the most complex of curves can be resolved into two simple movements. The sending pen, with which the original writing or drawing is done, is attached to the junction of two light rods. The farther end of each rod is attached to the end of a light crank fixed so that it can rotate or oscillate, after the manner of cranks, in the plane of the desk upon which the paper lies. All the joints mentioned are of the hinge nature, so that as the pen is moved about the rods turn, more or less, one way or the other, the two cranks. This simple mechanism, it will be observed, carries out very effectively the principle just mentioned, for it resolves the motion of the pen, no matter how complicated it may be, into a simple rotating motion of the two cranks. So the cranks turn this way or that as the draughtsman makes his picture, and it is very easy to arrange that their movement shall vary the strength of two electric currents, whereby we obtain electric currents varying in accordance with the movement of the cranks. This is done by making each crank operate a variable resistance or rheostat. When in its extreme position on one side the crank permits current to flow freely, but as it moves over to the other extreme position the resistance in the path of the current is increased. Such an arrangement is a common feature in electrical apparatus.