THE keys used in wireless work differ from those used in commercial wire telegraphy in being much heavier and stronger, so as to conduct and break the heavier currents without heating. Where very powerful currents must be broken the contacts of the key usually operate under oil or in the magnetic field of a pair of electromagnets. In other cases a small key is used to operate a heavy relay. In the former type a metal arm projects downward from the lever of an ordinary Morse telegraph key into a compartment filled with oil. The arm is provided with a platinum point which makes contact with another similar point on the bottom of the compartment, so that the break is made under oil and the key remains cool and does not arc. In the second form the same result is accomplished by a magnetic "blowout." The "blowout" is merely an electromagnet connected in series with the key and its poles at right angles to the contacts of the key. When the circuit is broken the magnetic field extinguishes the arc which tends to form between the contacts of the key.

An ordinary Morse key may be used with coils operated on batteries if they do not give more than a four-inch spark. Larger coils or transformers, especially if they are operated on the 110-volt current, require a heavier key. Where an ordinary Morse key is used with a 3-inch or 4-inch coil it is a very good plan to provide an auxiliary conductor, one end of which is fastened to the metal base and the other end to the adjusting screw on the back of the lever. A piece of flexible lamp cord is suitable for this purpose. This precaution saves the pivots of the key from heating and possible burning by heavy currents.

The most convenient method is to fit up an old Morse key in the manner shown in Fig. 67. The lever is extended by a piece of aluminum or brass 6 inches long and 5/16 inch thick. The exact shape and the dimensions are indicated in the illustration.

The extension lever is provided with a 1/2-inch round brass or nickel steel contact 7/8 inch long. The lower end of the contact is bored and tapped to receive a short machine screw. The other contact is a similar piece projecting downwards from a small arm fastened to the top of a standard 1/2 inch diameter and 2 inches long. The arm is a strip of brass 1/2 inch wide, 1/4 inch thick, and 1 1/2 inches long, fastened to the top of the standard with a machine screw. The brass standard is held in an upright position by means of a small machine screw which passes through the base. A wire connects with this screw to a heavy binding post mounted on the base. Connection is made with contact on the extension lever by means of a piece of heavy flexible lamp cord, which also runs to a binding post mounted on the base. The contact is insulated from the extension lever by means of two mica insulating washers and a bushing. Avoid running the connection to the base of the key so that the current must pass through the bearings. The contacts should be filed until they are perfectly flat and square across and make contact over their entire surface. When carefully constructed a key of this type will carry without heating almost any currents in reach of the amateur operator and which his induction coil or transformer will stand.

The plans and dimensions for a heavier key are shown in Fig. 68. A wooden pattern of the base and bearings is made and taken to a brass foundry where a casting in brass may be secured. The pattern should be of the same shape and size as the finished article represented in the figure. It is given a slight taper so that it may be easily withdrawn from the sand mold. The brass casting is finished up square and smooth by grinding on an emery wheel or by careful filing. The lever is a piece of brass rod 1/2 inch square and 6 1/2 inches long. One end is fitted with an ordinary hard rubber Morse key knob. The knurled nuts and thumbscrews are of the same size as those used on an ordinary key, and may be purchased from an electrical supply house.

The pivot is a piece of 3/16 inch round steel 1 inch long, and passes through the lever 1 1/2 inches from the rear end. The ends are ground or turned to sharp points and rest in bearings formed by boring a recess in the ends of the thumbscrews. The thumbscrews pass through the center of the bearing, standards 3/4 inch above the base. The lever is clamped to the center of the pivot by means of a small machine screw. A heavy piece of spring brass, S, 1/2 inch wide and 3 inches long, is fastened to the base at one end with a short 8-32 screw. The other end is bent upward so that it presses hard against the lower end of an adjusting screw, which passes through the lever 1 1/4 inches forward of the pivot. This spring serves as an auxiliary conductor and saves the key from carrying an excessive current through its bearings. By adjusting the thumbscrew, the tension of the key may be regulated to suit the operator. The contacts of the key are 1/8-inch round nickel steel alloy or pure silver, set in short 1/4-inch machine screws which fit into correspondingly threaded holes in the base and the lever. This makes the contacts easily removable for renewal. The lower contact is set in a longer screw than the upper one so that connection may be made to it. It is insulated from the base by a hard rubber brushing.

The contacts must be directly above one another and in perfect alignment or else they will pit and stick.