Arthur H. Bell

The relay used in the coherer circuit of a wireless telegraph receiver must be of the highest sensibility. The resistance of the magnet windings should be at least 1000 ohms for experimental work at short distances, and upwards to 5000 for practical field work. With the higher relays, the armature will operate or "throw over " with one volt throughout 30,000 to 50,000 ohms resistance, and it is this extreme sensibility which commends it to wireless specialists.

To the amateur who wishes to progress as far as possible in his researches with a limited expenditure of money, the cost of these relays would at first seem prohibitive, but it is the purpose of this article to explain the construction of a relay which is giving good satisfaction and which was completed for a reasonably small sum. The manner in which this relay operates reminds one of the vibrating attachment to a simple induction coil.

The electro-magnet consists of two square pieces of wood, 1 1/2" x 1 1/2" x 3/8" thick, each having a 3/4" hole through the centre. A bundle of fine annealed iron wires as fine as No. 24, or still finer if possible, 6" long and 3/4" in diameter is next procured, and one of the wooden squares glued securely on at each end, the iron core projecting outward a half inch at one end. This core is evenly wound with the finest magnet wire at one's command. The writer used No. 36 for a greater portion, and No. 34 to finish winding to the full capacity of the bobbin, having the same on hand, but it is best to use only one size, No. 36. It will be found that a weak cell of dry battery, not strong enough to ring a vibrating bell, will energize this core sufficient to attract iron filings through quite an air space. When completed the magnet is glued or screwed down to a base board of smooth wood about 4" x 8".

There is a kind of ribbon steel, similar to clock spring stock, which may be purchased of clock repairers, in small lengths. This is remarkably flexible, and should be procured in a perfectly straight strip 4" long and about 1/4" to 3/8" wide. This is to form the spring for the armature. At one end is soldered the armature, a soft iron disc about 1/8" thick and §" diameter. The other end is fastened to a small upright pillar post, placed in such a location on the base board that the iron disc is just opposite the magnet core, and 1/8" away from it. It will be found that this armature will respond to a very feeble current passed through the coil. The contacts are similar to those on an induction coil, only of course, the contact screw is on the other side of the vibrator, and touches to make contact only when the magnet is energized. It is a good idea to mount an adjustment screw to press on the outside of the spring vibrator, so as to closely adjust it in relation to the magnet and the contact screw. The contact on the contact screw must be a platinum point, and on the spring vibrator a little point of platinum is also necessary, for a perfect electrical contact is very essential.

There are four binding posts, two for the coil terminals and two for the relayed side. One binding post is connected to the vibrator and the other to the pillar contact post. In the construction of a relay as here described, the making of pivots and other difficult parts is avoided. It will also suggest other types, perhaps better adapted to the experimenter's particular requirements, as the study of wireless telegraphy is continued.