Page:Popular Science Monthly Volume 91.djvu/617

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�q Amateur • Electrician

��^nd Wireless Operator

���A coil of wire placed between record disks

��A Simple and Attractive Loading Coil

ORDINARY loading coils are often made in very compact form, as when wound between two hard rubber disks. Though very neat in appearance, such coils are hard to make without the aid of suitable ma- chinery for turning out the rubber disk ends, which are expensive if bought ready made. If the method of con- struction de- scribed in this article is used,

however, such coils can be easily made in a few minutes at a very low cost, and not only will they give the best of satisfaction, but they will present a very smart appearance.

The principal feature of this coil is embodied in the ends, which are small ten cent disk phonograph records. These can be bought for about five cents apiece second-hand at a record exchange shop, or in many cases at the 5 and 10 cent stores, wherever old records that do not sell well are reduced in price.

These composition disks are about $ l A i n - in diameter, and can be separated any desired distance by cardboard washers. The remaining space around the circum- ference of the cardboard and between the walls formed by the records is wound with fine wire, say about No. 28. The records can be turned so that the grooved sides face each other, thus leaving the smooth faces turned out and displaying the patent dates and numbers which are embossed on the bottom of the records.

The composition and cardboard disks are held together by binding posts passing

��through them near the center, or by several small copper rivets, arranged in a circle. These rivets may be used as con- tacts for a rotary switch, with which the inductance of the coil can be varied. After the holes for the binding posts and rivets have been made, and before the wire is wound on, all the disks to be used should be shellacked together in the proper positions, and dried under a flat weight of say 5 lbs.

The coil illustrated is a single step affair with only two binding posts. By making two separate windings, and fitting the coil with four binding posts, considerable ad- vantage is obtained over the single step type. With the latter scheme the instru- ment can be used as a straight tuning coil with two variations of inductance, or the separate windings can be so coupled as to form an inductive tuner. It can be used to increase the wavelength range of primary and secondary of any loose coupled tuning coil.

Instruments made in this manner must be handled with reasonable care, both in the making and use, as the composition ends, while almost as-strong as hard rubber of the same thickness would be, are too thin to be banged about indiscriminately. However, in case of breakage the cost of renewal is slight.— R. V. Clark.

��Using a Bicycle Pump for a Water Rheostat

A WATER rheostat can be quickly made from an old bicycle hand-pump. Remove the metal cap through which the rod passes, and substitute a plug of wood or cork.

Through the center of this plug a hole should be bored, its diameter being identical with that of the rod. Next remove the plunger disk and thoroughly clean the rod so it will be free from grease or rust. Wire leads should also be soldered to both casing and rod for electrical coh-

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