To secure the maximum efficiency of a coil it is essential that the vibrator or interrupter be exactly proportioned to primary current and service for which the coil is used. For small coils where simplicity of construction is desired, the ordinary spring vibrator is suitable, but careful designing and .workmanship are quite important for even this type. A quick, snappy break means a stiff spring, but one too stiff will not be self starting. On the other hand, a very flexible spring will vibrate slowly, making it unsuitable for wireless telegraphy. Some experimenting with springs both as regards length and flexibility will be advisable to secure the one most suitable. In general, this type should only be used on coils giving less than a two inch spark.

Induction Coil Making For Amateurs IV Vibrators an 199

Fig. 1.

The design for a simple spring vibrator is shown in Fig. 1. The posts A and B are about 3/8 in. diameter, and high enough to bring the spring S in line with the core of the coil. For both rigidity and appearance the bases of the ports should be larger, or 1 in. diameter, but this is not absolutely necessary. Washers can be put under the heads of the screws which are used to fasten the post to the base, and the posts will be rigid if not too frequently adjusted. The top ot the post A is slotted with a fine hack-saw to receive the end of the spring S. A hole is drilled | in. from the top of the post B to receive the contact screw C, and the post is then slotted. A hole is then drilled through the post to receive the set screw b at right angles to the hole for the contact screw.

The spring S is of spring brass 1/2 to in. wide, No. 30 to 24 gauge, and from 2 1/2 to 3 in. long, these dimensions covering coils giving from 1/2 to 1 1/2 in. sparks. A hole is drilled at one end to receive the screw a, and at the other end for the screw s, holding the hammer

Induction Coil Making For Amateurs IV Vibrators an 200

Fig. 2.

head. At the point where the contact screw c touches the spring, about midway between the post, and hammer, a piece of sheet platinum is soldered, or a piece of platinum wire 1/8 in. diameter and 1-16 in. thick may be used. The contact screw should also have a platinum point. In soldering on the platinum, first clean the surface with a fine file or emery cloth; then with a blow-pipe or torch, melt a drop of hard brazing solder and, keeping the solder fluid with the torch, drop on the platinum and immediately remove the flame. Use only enough solder to firmly attach the platinum. The contact screw c should be about 1 1/2 in. long and 3-16 in. diameter, and have a large head at the outer end to facilitate fine adjustment. In testing the efficiency of vibrators, care must be used to learn if troubles arise from the vibrator, condenser, or weak or excessive currents in primary circuit, as the improper operation of the vibrator may result from any of these sources.

Another form of vibrator with a double spring has decided advantages over the one just described. From its action can be secured a long " make " and a short, quick " break, " thus obtaining thorough saturation of the core and a maximum spark. The rate of vibration can also be varied to a small extent. It is suitable for use with coils giving up to 8-in. spark, but a motor-driven, mercury-dip break is advised for coils giving a 4-in. spark, or over. The design of the double spring vibrator is shown in Fig. 2. The hammer II, is supported by a brass collar C, to which is riveted the spring A. The lower end of the spring is riveted to the U-shaped brass base, which is fastened to the wood base of the coil by a screw.

Induction Coil Making For Amateurs IV Vibrators an 201

Fig. 2.

Near the lower end of the spring A is brazed a small nut N, which is threaded on the adjusting screw S. The end of the adjusting screw is threaded through a hole in the other arm of the brass base, and has a checknut M for binding fast when adjusted. A second spring B is riveted to an L-shaped spring base and has near the lower end a bushing of hard rubber B, fitting over the adjusting screw S. At the upper end is brazed a brass nut, which is used to more securely hold the arm K, one end of which projects to the front and is tipped with platinum, and the other end passes through a rather larger hole in the spring A and collar C, and is fitted with a threaded check nut L. The hole in the spring and collar should be large enough to prevent the arm from binding. The check nut L should fit tightly, and may also have a check nut outside for binding same to position.

The contact screw E is supported by the brass post I) and has a check nut F. The end is tipped with platinum to prevent sparking. The platinum point of the arm K rests upon the platinum tip on the end of the contact screw E, which allows the primary current to flow around the core, thus attracting the hammer

II. As the hammer moves towards the core with accelerating speed the collar C closes against the nut L, and forces the arm K to "break" from the contact screw E, which occurs just previous to the striking of the hammer against the core. The value of this design of vibrator arises from the momentum attained by the hammer before causing the break, which is quick and sharp. As the hammer recedes from the core after the " break " the contacts are quickly united and remain so until the hammer completes its swing, and again returns under the impulse of the spring A, and the at-ti'active force of the core.

The two forms of vibrators described are suitable only when the variation of the vibrations is within narrow limits. For experimental work or wire less telegraphy, where a wide range or a very rapid rate of vibrations is necessary they will not answer, and the motor driven mercury dip interrupter is more desirable. Such a device is not difficult to make, nor very expensive when made as shown in Fig. 3. An ordinary toy motor is mounted on a wooden block of suitable height, which in turn is supported upon a base board holding a short glass bottle with a wide mouth. Four binding posts are mounted, two on one side near the bottle, and two on the end nearest the block.

The motor is run by a battery of two dry cells and a rheostat is mounted on the outer surface of the block and connected into the motor battery circuit. By means of the varying resistances the speed of the motor can be varied over a wide range and adjusted to any required rate of interruptions for the coil. The bottle is partly filled with mercury, which is covered with a thin layer of turpentine or crude petroleum. The fixed lead B is a piece of heavy gauge copper wire-running from one binding post, over the top of the, bottle and into the mercury.

The vibrating terminal A is connected to the other binding post by a spiral of flexible wire, which is attached, at the outer end of the spiral, to the block holding the motor. This wire is soldered to the vibrator with soft solder to give good electrical contact. The movement of the vibrator is obtained by means of a brass box D, leaving a long slot which works on a stud put into the outer face of the small pulley on the motor. This stud is simply a short piece of brass wire, driven into a hole drilled in the pulley, or may be a round head, brass machine screw and the hole in the pulley threaded to receive it. The latter way is preferable, as the head of the screw serves to hold the box in position.

The upper and lower parts of the vibrator are brazed into holes drilled in the box. The box is cut out of a small piece of brass rod ; the slot is formed by drilling holes and finishing with a small, flat file. The box and pulley are shown in enlarged view in Fig. 3. The vibrator should have a movement of about | in., and to obtain this it may be necessary to substitute for the motor pulley a disk of brass large enough to give room for the stud, which should be 3-16 in. from the center of the disk.

The bearings for the vibrator are made of strips of heavy brass, the ends of which are secured under the ends of the armature bearing on the motor. These strips are bent to an L shape, and holes are drilled in the outer ends to receive the vibrator with a sliding fit. To give a heavier bearing that will not quickly wear out, small brass discs are brazed over the holes, and bored out to a sliding fit for the vibrator. Graphite should be used as a lubricant for these bearings.

The vibrator is also given a short point at the end entering the mercury to prevent spattering. The duration of the "make" is regulated by the depth of the mercury; the greater depth the point enters the mercury, the longer the current flows through the primary. It is necessary that the point entirely clears the mercury on the up stroke, to ensure a good break-A little experimenting will quickly show the necessary adjustment. In place of changing the depth of mercury, alterations of the " make" can be obtained by placing pieces of cardboard under the bottle. To prevent the bottle from moving bore a shallow hole in the base board to hold it in place. This type of interrupter should not, for convenience sake, be mounted on the coil base, but may be made a separate fixture together with the condenser.