One factor in determining the intensity of an induced current is the distance the wire in which it flows is from the source of induction. A telephone put in circuit at the end of the two wires that are thus laid together will be practically free from the effects of induced currents that are set up by the working of contiguous wires--for this reason: Whenever a current is induced in one of the slack-twisted wires it is induced in both alike; the two impulses being of the same polarity meet in the telephone, where they kill each other. In order to have a perfect result we must have perfect conditions, which are never attained absolutely, but nearly enough for all practical purposes. In the early days of telephony great difficulty was experienced in using a single wire grounded at each end in the ordinary way, if it ran near other wires that were in active use. As time passed on and the electric light and electric railroad came into operation these difficulties were immensely increased, till now in large cities the telephone companies are fast being driven to the double-wire system, which will soon become universal for telephonic purposes the world over, except perhaps in a few country places where there is freedom from other systems of electrical transmission. To successfully work the telephone and telautograph through the same cables, these protective devices against induction must be very carefully provided and maintained. CHAPTER XX. SOME CURIOSITIES. Until within recent years it was never supposed that a sunbeam would ever laugh except in poetry. But the modern scientist has taken it out of the realm of poetry and put it into the prosy play of every-day life. The Radiophone, invented by A. G. Bell, is an instrument by which articulate or other sounds are transmitted through the medium of a ray of light. It has as yet no practical application and has never gone beyond the experimental stage, but as a bit of scientific information it is very interesting. If we introduce into an electric circuit a piece of selenium, prepared in a certain way, its resistance as an electric conductor undergoes a radical change when a beam of sunlight is thrown upon it. For instance, a selenium cell, so called, that in the dark would measure 300 ohms resistance, would have only about 150 ohms when exposed to sunlight. This amount of variation in a short circuit of low resistance would produce a considerable change in the strength of a current passing through it from a battery of a given voltage.