318
��Popular Science Monthly
��Strain Insulator Construction for Aerial Lines
��THE accompanying illustrations show a method of constructing strain insu- lators which answer the purpose admirably. Procure, for each insulator required, a piece of wood ij/g or 2 ft. long and 2 in. square. Chuck each piece in a wood lathe and turn them to 2 in. in diameter. At intervals of 3 in., turn V-shaped grooves A Fig. I, i}4 ii^- deep. Remove from the lathe and drill )^-in. holes B in the ends and trim them flat with a draw shave as shown. Soak the wood for at least an hour in a molten mixture of equal parts paraffin and resin, to which has been added enough black asphalt to color. Allow the mixture to penetrate the wood as far as possible. Cut circles 123^ in. in diameter from card- board. Holes C Fig. 2, i^ in. in diameter are cut out, and one-third of the remaining surface, represented by the dotted line, is cut away. These cardboard segments are
���An insulator made of wood and cardboard disks then thoroughly soaked in paraffin and resin
then boiled in the above mixture until com- pletely impregnated. Coil them around the grooves, allowing the edges to overlap 3^ in., and fasten with thread as shown in Fig. 3. The completed insulator is now immersed in the insulating mixture until all cracks have been filled and air bubbles
��expelled, then removed and allowed to cool. It is practically impossible for a film of moisture to cover this insulator, even during damp weather. — H. W. Offins.
Making a Tuning Coil Slider from a Fountain Pen Clip
AVERY good tuning coil slider can be made from a fountain pen clip and a small block of wood with a hole in the lower
���A slider, made from simple materials, which passes easily over the wires on a tuning coil
part of it for the slider rod. The sides A of the clip B should be bent into a square form so that they will fit around the block as shown at C. Tack the clip on the block, making sure that the bottom of the clip will touch the slider rod when it is mounted on the tuning coil. This is very important because the contact is made at this place. The arm with the knob on the end should be bent down until it touches the coil. A knob may be fastened on top of the block for convenience. — Clarence Hunt.
��Detecting Defects in Castings with Magnets
DETECTING the presence of internal defects in iron and steel castings is accomplished by a current which is supplied from a small alternator to the primary winding of an induction coil, and which induces in the secondary winding a current which is passed through coils of two horse- shoe magnets, mounted at a fixed distance apart and movable to and fro over the surface of the casting. The fields of both magnets will be affected uniformly if the structure is homogeneous. Defects, how- ever, will disturb the lines of force from the nearest magnet. In a vibrating sounder, connected with a secondary coil on the magnet, the tone produced will differ in pitch from that given out from the cor- responding sounder connected with the other magnet. Amplification of sound is secured by microphone attachments en- abling the observer, by means of telephone receivers, to detect the locality of hidden flaws as the magnets are moved to and fro over the castings.
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