Another form of coherer is shown in Fig. 26. This is made of a small piece of glass tube, two rods that will accurately fit in the tube, some nickel filings, two binding-posts, and a base-block three inches and a half long. The two binding-posts are mounted on the block, and through the holes in the body of the posts the rods are slipped. They pass into the tube, and the blunt ends press the small mass of filings together, as shown in the drawing. By means of the binding-posts these coherer-rods may be held in place and the proper pressure against the filings adjusted; then maintained by the set-screws. The nickel filings may be procured by filing the edge of a five-cent piece. Obtain a few filings from the edge of a dime and add them to the nickel, so that the mixture will be in the proportion of one part silver to nine parts nickel. This mixture will be found to work better than the iron filings alone. The aerial and ground wires are made fast to the foot-screws of the binding-posts, and the base on which the coherer is mounted may be attached to a table or ledge on which the other parts of the receiving and recording apparatus are also installed.
Another form of coherer is shown at Fig. 27. This is constructed in a somewhat similar manner to the one just described. A glass tube is provided with two corks having holes in them to receive the coherer-rods. Two plugs of silver are arranged to accurately fit within the tube, and into these the ends of the coherer-rods are screwed or soldered. Between these silver plugs, or terminals, the filings of nickel and silver are placed, and the rods are pushed together and caught in the binding-posts. The aerial and ground wires are made fast to the foot-screws of the posts.
For long-distance communication it is necessary to have a condenser placed in series with the sparking or sending-out apparatus. (See the type of condenser described and illustrated in chapter iv., page 72.)
An astatic galvanometer is also a valuable part of the receiving apparatus, and the one described on page 111 will show clearly the presence of oscillatory currents by the rapid and sensitive deflections of the needle.
For local service, where a moderately powerful battery is employed, a telegraph-key, such as described on page 190, will answer very well, but for high-tension work, where a powerful storage-battery or small dynamo is employed, it will be necessary to have a non-sparking key, so that the direct current will not form an arc between the terminals of a key. Most of the keys used for wireless telegraphy have high insulated pressure-knobs, or the make and break is done in oil, so that the spark or arc cannot jump or be formed between the points.