Page:Heroes of the telegraph (IA cu31924031222494).djvu/268

 have long had sore reasons for regarding a 'bad contact' as an unmitigated nuisance, the instrument of the evil one, with no conceivable good in it, and no conceivable purpose except to annoy and tempt them into wickedness and an expression of hearty but ignominious emotion. Professor Hughes, however, has with a wizard's power transformed this electrician's bane into a professional glory and a public boon. Verily there is a soul of virtue in things evil.

The commonest and at the same time one of the most sensitive forms of the instrument is called the 'pencil microphone,' from the pencil or crayon of carbon which forms the principal part of it. This pencil may be of mercurialised charcoal, but the ordinary gas-carbon, which incrusts the interior of the retorts in gas-works, is usually employed. The crayon is supported in an upright position by two little brackets of carbon, hollowed out so as to receive the pointed ends in shallow cups. The weight of the crayon suffices to give the required pressure on the contacts, both upper and lower, for the upper end of the Pencil should lean against the inner wall of the cup in the upper bracket. The brackets are fixed to an upright board of light, dry, resonant pine-wood, let into a solid base of the same timber. The baseboard is with advantage borne by four rounded india-rubber feet, which insulate it from the table on which it may be placed. To connect the microphone up for use, a small voltaic battery, say three cells (though a single cell will give surprising results), and a Bell speaking telephone are necessary. A wire is led from one of the carbon brackets to one pole of the battery, and another wire is led from the other bracket to one terminal screw of the telephone, and the circuit is completed by a wire from the other terminal of the telephone to the other pole of the battery. If now