Arthur H. Bell.

Amateurs interested in Wireless Telegraphy are often puzzled as to the best relay to use in connection with their receiving apparatus, and as a sensitive relay is very essential, I will briefly describe the different kinds of relays that are on the market or which may be constructed at home, and give the reasons why one type is superior to another.

Nearly every amateur is familiar with the telegraph relay, so common to wireless telegraphy. This type of relay is a neutral or simple electro-magnetic relay, designed with magnets exactly like the magnets of a vibrating door bell, only finer wire is used in the coils. In this type of relay it matters not as to the direction of the current in its course to the relay, the only requisite being that there be enough "juice" passing into the relay to magnetize the cores and attract the armature, and in the ordinary 150 ohm telegraph relay, by careful adjustments, one can make the armature throw over with the current from one cell of dry battery. But anything less than 1 1/2 volts will not do this every time. So, for service in a coherer circuit where, under normal conditions there is not sufficient current passing into the relay to cause it to operate, and where the current increases considerably when the coherer is affected by the Hertzian waves, the telegraph relay is a good instrument for short distances only, say for one to four miles at the most.

There is a relay, however, used universally in telegraphy which one does not find in every small office, but which plays a very important part in rapid commercial business where one telegraph circuit is handling two or more messages simultaneously.

This apparatus is the polarized relay, unquestionably the most sensitive instrument to be obtained at a moderate price. By examining the illustration the reader will note that there is a permanent magnet of horse-shoe type, having at one pole a pair of wound electro-magnets exactly like the magnets on the ordinary relay. As these magnets have soft iron cores, it should be understood that the magnetism of the steel permanent magnet saturates them and makes both of them of the same polarity as the tip of the steel magnet to which they are fastened; that is to say, if the bottom pole of the permanent magnet is south these poles of the electro-magnet are south. To the other pole of the permanent magnet is pivoted, sensitively, a soft iron bar which takes, of course, the same polarity as the pole to which it is affixed. So we have a movable north pole playing between the two poles of the electro-magnets, and bear in mind this point, this condition of affairs occurs only when the current of electricity passing through the coils from the battery, is not of right direction of flow to change the polarity of the coils and cause the armature to swing over because of this change. So we note that the operation of a polarized relay does not depend on attraction; that is, sufficient generation of lines of force to pull a soft iron armature towards the poles of an electro magnet, as with the first type of relay, but depends absoutely on changes in direction of current.

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In wireless work there is sufficient tension put on the pivoted armature to cause it to hug the pole opposite the contact point, and the battery is poled right by testing, so that when applied directly to the windings, the armature swings over instantaneously and makes contact. With this relay and a dry cell in circuit with the filings coherer and a small set of adjustable resistance cut into the circuit, by close adjust" ment the permanent magnetic effect of the relay can be opposed with just enough poled energy, when the coherer operates, to make a very delicate and rapid contact.

Any amateur who wishes to experiment with a polarized relay will find the 600 ohm bell used by telephone companies a very satisfactory instrument and very easily constructed for wireless work.