Oscar F. Dame

Every student of wireless telegraphy is familiar with the story of the first coherer, with which electrical disturbances in the air were noted by the operation of a relay and the ringing of a bell.

This same coherer principle enters into the receiving apparatus of many wireless systems. While the scheme is an old one, there is still an excellent opportunity for experiments in this line, and apparatus-built according to the accompanying specifications should be a fairly accurate recorder of the excess of electrical impulses in the air which usually precede a thunder storm.

Fig. 1 represents a coherer designed especially for receiving high voltage disturbances, and immune from arc ligh and trolley spark waves. The glass tube may be purchased at any drug store for a few cents. The diameter of the tube is about 1/4 in. The tube is heated over an alcohol or gas flame and bent to a V form. A sack is also drawn out on one side near the top to hold extra filings. The two electrodes are of polished heavy copper wire.

This coherer is not exhausted, and the amount of filings at the electrodes may be regulated by drawing from the supply at the sack. The filings are of clean cast iron, as fine as can be made. Any relay suitable for wireless telegraphy, many of which have been described in back numbers of Amateur Work, will operate with this receiver. The electro-magnet is about the size of one coil in an electric door bell, but should be wound with finer wire, No. 24 being a good size. The purpose of this magnet is to serve as a decoherer when the relay operates. The coherer should be supported by two wooden pillars mounted on a base-board. These can be made from meat skewers. A 2 1/2 in. door bell serves for a signal in place of a sounder. The aerial wire and ground wire should be constructed the same as those of a simple wireless circuit.