Arthur H. Bell
Amateurs desiring a sensitive relay for their wireless receiver at moderate cost, have been unable to find anything in the open market cheaper than the 150 ohm telegraph relay of general commercial use.
This relay costs in the vicinity of $7 and is not exactly suitable for use in the coherer circuit because of its sluggishness. Polarized telegraph relays of high resistance winding are not carried in stock by supply houses, and the cost of a special one, built to order, would be over $15. Thus it is that the amateur, search-ing the apparatus field for a satisfactory combination of electro magnet and permanent magnet with which to make a relay, happens across a very common telephone part - the polarized bell or polarized ringer, as it is often styled.
As will be seen by the illustration, Fig. 1, the polarized ringer has two electro-magnet coils, wound with very fine wire. The resistance of the wire often reaches 1000 ohms to the coil, making 2000 ohms to to the pair. In fact, there are standards of resistance for these windings, namely, 80, 500, 1000, 1500, 2500 ohms per set. The purpose of these ringers is patent to all who have read previous articles in this magazine on telephone systems, and as all reliable telephone instruments are equipped with them, every reader will have an opportunity to observe their operation in actual telephone usage.
For our purpose, however, the value of a ringer in the telephone field will not be considered. Any manufacturer of coils and telephone parts can supply these either with or without the two gongs, for less than $4, the price depending upon the resistance and quality of the winding and the general finish of the parts.
It will be noted that there is an armature pivoted over the pole pieces of the magnets, and a permanent steel magnet with one pole fastened to the yoke of the electro-magnets and the other pole an inch above the armature. Because of this permanent magnet the ringer is polarized and the armature kept in stress and acting very inert when the coils are energized. For example, a dry cell of battery connected to the coil terminals will cause the armature to tilt in a direction according to the direction the current is sent through the windings.
Here, therefore, is a simple polarized relay, readily adaptable to wireless service by connecting the coil wires to the binding posts for the coherer circuit, and placing a contact point on the bell hammer rod and a contact post on the baseboard with which to make contact when the armature swings over. The connections are exactly as with the familiar telegraph relay.
It must be understood, however, that the pivots of these $4 ringers were not calculated for the exceedingly sensitive work of wireless telegraphy, and all the parts of the ringer are not adaptable to our purpose* The bell coils and the yoke bar to which they are fastened should be kept intact. The armature and hammer and the pivot supports should be removed. The steel magnet is neither long enough nor high enough above the coils. It will be found useful some day for experimental purposes and should not be thrown away.
Another magnet of steel (Jessops) out of 1/8 x 3/4 stock should be forged into horse-shoe shape, with poles the same distance apart as on the magnet removed. A hole should be drilled at each end, one for fastening to the coil yoke, and the other to support a set of bearings for the armature. The piece should then be magnetized. Fig. 2 shows the general shape of this magnet.
In selecting style of bearings the knife edge is undoubtedly the easiest to make, and fully as efficient as ordinary jewels. The sketch in Fig. 2 will illustrate how the bearings of this relay are formed on the top of the magnet, which is polished with a fine file, a niche cut with the same and smoothed on an oil stone. The armature and the supporting rod must be of soft and very thin, light pieces of iron.
Out of the bell armature which was removed, must be fashioned two pieces, as in Fig. 3, to be adjustably fastened with machine screws to the poles of the electro magnets. In use these pole pieces are just far enough apart for the armature to swing in between.
The maker may use his own judgment as to the best method of connecting a contact point to this armature, the facilities for doing this being different in each individual case, but one way in which this can be done is to solder a point of platinum to the lower end of the pivoted pendulum, which contact strikes against another contact spring attached to a wooden or metal pillar post, which is affixed to the base-board near the magnet coils. (See Fig. 4.) It will be noted in Fig. 4 that when the armature swings over and a contact is made, that a complete circuit is made through the sounder.