Henry C. Wall

Quite frequently the strength of battery current in use on a given piece of electrical apparatus has to be varied or regulated to meet certain conditions. Unless the amateur possesses a set of resistance coils arranged for easy handling, the task is likely to be a difficult one, especially when the exact amount of resistance to be applied is to be ascertained only by experiment.

The writer not long ago, in connection with wireless telegraph experiments, felt the need of a suitable device with which he could increase or decrease gradually the flow of current in his coherer circuit, and as the device, when completed, gave good satisfaction, a description of it is here given for the benefit of Amateur Work readers. A piece of round hard wood, about one inch and a half in diameter and twelve inches long was procured. Such pieces may be readily purchased at furniture stores where portiere poles are sold.

A small metal binding post was fastened to one end of the piece. A piece of No. 30 German silver resistance wire was soldered to the binding post. If the German silver wire is not to be had in your vicinity the same gauge of bright steel wire will suffice. Fastened at the binding post along with the wire is one end of heavy silk thread, or black linen might answer. The thread and the wire are to be wound around the wooden cylinder, parallel with one another, so that at all times an insulating thread is between the wire turns. In this way, by winding evenly, a good amount of resistance wire may be placed in a space. The entire winding is then given a very thin coat of shellac and when thoroughly dry, the wire brightened by rubbing with an old piece of fine sand paper.

The amateur will now observe how this coil, when inserted in a circuit, may be used as a veritable resistance, to be determined by the portion of the coil that is included in the circuit.

It is necessary, therefore, to provide a sliding contact to slip back and forth as desired. This may be made of a block of wood with a 1| in.hole in it, or of fibre tubing. Inside the tubing is placed a strip of spring brass, bent inward in such a position that the wire wound on the cylinder is rubbed by one end of the brass strip as the "rider "is moved back and forth, and to this rider is soldered another binding post. Whenever this device is introduced in a circuit, the amount of current is varied according to the amount of resistance; that is, the number of resistanoe wire turns included in the circuit.

Instead of using a cylindrical rod, the amateur might cut out a thick ring of pine or other wood and use one-half, one-quarter, or as much of the curve as desired, to wind on, and instead of the riding contact described above, arrange a swinging wipe contact similar to the switch arm used on ordinary single piont switches. This would entail additional work, however, and unless the amateur is provided with tools suitable for cutting out the circle, it is advisable to follow the simpler method.