Page:Heroes of the telegraph (IA cu31924031222494).djvu/229

 every signal, thus pressing on the plumbago, and by reducing its resistance varying the current in the local circuit. According as the main line current was strong or weak, the pressure on the plumbago was more or less, and the current in the local circuit strong or weak. Hence the signals of the local receiver were in accordance with the currents in the main line.

Edison found that the same property might be applied to regulate the strength of a current in conformity with the vibrations of the voice, and after a great number of experiments produced his 'carbon transmitter.' Plumbago in powder, in sticks, or rubbed on fibres and sheets of silk, were tried as the sensitive material, but finally abandoned in favour of a small cake or wafer of compressed lamp-black, obtained from the smoke of burning oil, such as benzolene or rigolene. This was the celebrated 'carbon button,' which on being placed between two platinum discs by way of contact, and traversed by the electric current, was found to vary in resistance under the pressure of the sound waves. The voice was concentrated upon it by means of a mouthpiece and a diaphragm.

The property on which the receiver was based had been observed and applied by him some time before. When a current is passed from a metal contact through certain chemical salts, a lubricating effect was noticeable. Thus if a metal stylus were rubbed or drawn over a prepared surface, the point of the stylus was found to slip or 'skid' every time a current passed between them, as though it had been oiled. If your pen were the stylus, and the paper on which you write the surface, each wave of electricity passing from the nib to the paper would make the pen start, and jerk your fingers with it. He applied the property to the recording of telegraph signals without the help of an