By means of a switch located in each subscriber's office the wires may be switched from a telephone to a telautograph, or vice versa, in a moment of time. By this arrangement a man may do all the preliminary work of a business transaction through the telephone, and when he is ready to put it into black and white switch in the telautograph and write it down. For ordinary exchange work this is undoubtedly the true way to use the telautograph, because one system of wires and one central-station system will answer for both modes of communication, and in this way an enormous saving can be made to the public. There is no question in the mind of any one who is familiar with the operation of both the telephone and telautograph but that some day they will both be used, either in the same or separate systems, as they each have distinctly separate fields of usefulness,--the telephone for desultory conversation, the telautograph for accurate business transactions. The question may arise in the minds of experts how the two systems can be worked in the same set of cables, and this leads us to discuss the phenomena of induction. Every one who has listened at a telephone has heard a jumble of noises more or less pronounced, which is the effect of the working of other wires in proximity to those of the telephone. If, when a Morse telegraph instrument is in operation on one of a number of wires strung on the same poles, we should insert a telephone in any one of the wires that were strung on the same poles or on another set of poles even across the street, we could hear the working of this Morse wire in the telephone, more or less pronounced, according to the distance the wire is from the Morse circuit. This phenomenon is the result of induction, caused by magnetic ether-waves that are set up whenever a circuit is broken and closed, as explained in Chapter VI. The telephone is perhaps the most sensitive of all instruments, and will detect electrical disturbances that are too feeble to be felt on almost any other instrument, hence the telephone is preyed upon by every other system of electrical transmission, and for this reason has to adopt means of self-protection. It has been found that the surest way to prevent interference in the telephone from neighboring wires is to use what is called a metallic circuit--that is to say, instead of running a single wire from point to point and grounding at each end, as in ordinary telegraph systems, the telephone circuit is completed by using a second wire instead of the earth. As a complete defense against the effects of induced currents the wires should be exactly alike as to cross-section (or size) and resistance. They should be insulated and laid together with a slight twist. This latter is to cause the two wires so twisted to average always the same distance from any contiguous wire.