So sensitive is this “interrupter” system that as many as sixteen messages can be sent simultaneously, which means that a single wire is conveying from 500 to 800 words a minute. We can easily understand the huge saving that results from such a system; the cost of instruments, interrupter, &c., being but small in proportion to that of a number of separate conductors. The word-sending capacity of a line may be even further increased by the use of automatic transmitters able to work much faster in signal-making than the human brain and hand. Sir Charles Wheatstone’s Automatic Transmitter has long been used in the Post-Office establishments. The messages to be sent are first of all punched on a long tape with three parallel rows of perforations. The central row is merely for guiding the tape through the transmitting machine. The positions of the holes in the two outside rows relatively to each other determine the character of the signal to be sent. Thus, when three holes (including the central one) are abreast, a Morse “dot” is signified; when the left-hand hole is one place behind the right hand, a “dash” will be telegraphed. In the case of a long communication the matter is divided among a number of clerks operating punching machines. Half-a-dozen operators could between them punch holes representing 250 to 300 words a minute; and the transmitter is capable of despatching as many in the same time, while it has the additional advantage of being tireless. The action of the transmitter is based upon the reversal of the direction or nature of current. The punched tape is passed between an oscillating lever, carrying two points, and plates connected with the two poles of the battery. As soon as a hole comes under a pin the pin drops through and makes a contact. At the receiving end the wire is connected with a coil wound round the pole of a permanent bar-magnet. Such a magnet has what is known as a north pole and a south pole, the one attractive and the other repulsive of steel or soft iron. Any bar of soft iron can be made temporarily into a magnet by twisting round it a few turns of a wire in circuit with the poles of a battery. But which will be the north and which the south pole depends on the _direction_ of the current. If, then, a current passes in one direction round the north pole of a permanent magnet it will increase the magnet’s attractive power, but will decrease it if sent in the other direction. The “dot” holes punched in the tape being abreast cause first a positive and then a negative current following at a very short interval; but the “dash” holes not being opposite allow the positive current to occupy the wires for a longer period. Consequently the Morse marker rests for correspondingly unequal periods on the recording “tape,” giving out a series of dots and dashes, as the inker is snatched quickly or more leisurely from the paper.