Page:Popular Science Monthly Volume 49.djvu/236

222 metallic springs of exquisite delicacy. At each perforation the springs touch and the current takes its way through the wire. At the receiving station the delay involved in the arousal and action of electro-magnets is abolished. The current instant by instant writes its message on a moving ribbon of paper sensitized so as to change color under an electric flow. This instance is typical of what ingenuity can do when electricity is added to its armory. A task is divided between an operator and an automatic machine in such wise that intelligence is allotted only that part for which intelligence is required, while for the remaining part the utmost speed of electrical and chemical action is invoked—a pace which in this particular case sixtyfold outstrips the most dexterous manipulation.

Another means by which inventors have expedited telegraphy has been by transmitting several messages simultaneously over a single wire. Of these multiplex systems certain are synchronous in principle and seem to have suggested to Prof. Elisha Gray his telautograph, an instrument that imitates exactly the motion of a pencil, in say Boston, by the motion of another, in say Baltimore, reproducing with equal facility either handwriting or outline drawing. To understand the principle involved, let us glance at an everyday application of electricity in keeping scores of clock pendulums, no matter how far apart, in perfect step. If two pendulums at right angles to each other are attached to a moving pencil their motions may be communicated to a distance by two currents which actuate two pendulums in control of a second and copying pencil. The electric clock at which we have just been looking can, if we please, be sealed in a glazed box, secure from dust and dampness. Here opens a fresh path to the inventor who wishes to avoid the resistance or leakage entailed when a rod moves through a slot or a stuffing box. It is often of cardinal importance that a bit of metal at rest should throb with a pulse strong enough to do severe drudgery or tell a tale which otherwise would go untold. If an engineer wishes to know how much heat wastes itself through the walls of a steam cylinder, his question is answered through a motionless wire attached to a delicate metallic thermometer buried in the cylinder's mass. In experimenting with new alloys the same method informs the chemist of changes of temperature at the core of his crucible, changes often abrupt and transient and at times denoting qualities he seeks to detain or reproduce. In a very different domain of exploration the engineer uses the telephone to expose perilous defects in metal beams.

As we prove when we unhook a telephone, or lift an incandescent lamp, electricity readily traverses a flexible wire: this unbars a fresh resource to invention. To-day rock drills, coal