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. 



