Arthur H. Bell.

In wireless telegraphy one of the chief difficulties which operators have had to contend with is the securing of a sensitive and reliable receiver of the wave impulses from the transmitting station. This is especially true where the distances between stations have been at all great, the energy from large coils not being adequate to operate the coherer, which has been the form of wave detector usually adopted by amateurs.

The detector here described is of quite a different character; the principles involved in its operation will not here be considered. It is not difficult to construct if care be used in the handling of the delicate wire which forms the contact with the electrolyte. The materials, with possibly the exception of this fine wire, known as Wollaston wire, are easily obtainable in any large town or city. Wollaston wire is made by silver plating fine platinum wire, then drawing down through dies, thus greatly reducing the diameter of the core of platinum wire. The silver plating is then taken off by suitable acids leaving the minute platinum core exposed. As it is a rather difficult matter to handle this wire, the amateur is advised to first use No. 40 gauged platinum wire until sufficient skill in its use has been attained to use the other to good advantage.

To make the detector, obtain a block of solid brass about 4 in. square and 1/2 or 3/4 in. thick. This weight is needed to give solidity to the device and reduce vibration to a minimum. Also obtain a piece of brass rod 6 in. long, | in. thick and 1/2 in. wide. This is bent to the shape shown in the illustration by heating in a blast torch until ductile and easily bent. Holes are drilled and tapped in the feet for 1/4 in. brass screws, the feet being filed off to a good contact with the base; or this frame may be brazed to the base.

In the center of the top, drill and tap a hole for a 1/4 in. thumb srew with knurled head and fine pitch. Get as fine a thread us possible, as the adjustment is made with this screw, and a fine thread is desirable. In the lower end of this screw drill a hole for a No. 8 gauge brass wire, a short piece being fitted to this hole and the lower end slit by sawing with a fine fret saw. The burr made by slitting is removed with a fine file and one end of a short piece of the fine platinum wire is placed in this slit, which is then closed tight with nippers.

Obtain a piece of dynamo-brush about 1/2 in. square; round would be better but not bo easily obtained. French arc-light carbon of a very fine grain will answer. Saw off a length of about 1/2 in. and coat the outside with several coats of shellac, using care not to get any on the top end. In the top drill a 1/8 in. hole about 3/8 in. deep. Mount this piece of carbon under the brass frame so that the platinum wire will be exactly in the center of the hole. This is done by coating the bottom of the cup with shellac, and before drying place it upon a piece of hard rubber which is attached to the brass block in the same way.

Fig. 1

The leads are next attached, one to the carbon cup and one to the adjusting screw. Place a drop of soldering flux upon one edge of the top of the cup, als0 upon the end of a piece of double covered magnet wire, No. 18 gauge, then with a small soldering iron drop a bit of solder upon the wire which has been previously placed upon the cup. It is also advisable to make a coil in this lead of about a dozen turns around a pencil, about 3 in. from the cup.

The other lead is soldered to the top of the adjusting screw and should have a similar coil of about a dozen turns. These turns of wire are to ease any undue strain which might be brought upon the leads by the operator when handling.

Fig. 2.

The connections are shown in Fig. 2, in D which is the wave detector, B a local battery of one dry cell, C a variable resistance coil of about 50 ohms resistance, R a telephone receiver, G the ground and A the aerial. The construction of a resistance coil will be described in a subsequent article.

In making this detector, it will be advisable, to save breakage of the platinum wire, to put it in place after all the work is done and the detector in position for operating. A pair of long nose pliers of small size will be very servicable in handling the wire. When all adjustments are made the hole in the carbon cup is nearly filled with a solution of nitric acid and distilled or rain water; 1 part acid to 4 parts water. The platinum wire should enter the solution about 1/8 in.

A little experimenting will be necessary to determine the correct resistance of the coil G. Too much current will cause the platinum wire to be eaten away rapidly; too little will reduce the sensitiveness of the detector. The platinum wire should be removed from the electrolyte when through using.