Radio broadcast .. (1922-30)

The Right Insulator in the Right Place The Best Kinds to Use for Cabinets, Coils, and Condensers By PAUL McGINNIS THE builder of a radio set thinks first of all of conductors, proper sizes of wire, and proper connections and is likely to overlook the fact that without insulators there could be no conductors. He knows in general that a rubber composition is better than wood for making panels, but he may ruin an otherwise good set by overlooking smaller details of insulation in out-of-the-way places behind the scenes. The only difference between conductors and insulators is in their resistance to electrical current. In the class of materials called conductors, the cohesion between the atoms and their electrons is considered to be overcome more easily by electrical pressure than in the materials called insulators. The vast difference in the common kinds of insulation is particularly important in radio where alternating currents of high frequency are used, and especially in transmitting apparatus where high voltage is employed. The problem of covering coil windings with a proper insulating compound is one which presents itself to the amateur at an early stage in his radio progress. He wonders whether he should use paraffin or shellac. The paraffin will rub off more easily, he thinks, and decides to use ellac, perhaps, because it is a "good inShellac is in fact a " good insulator " since one cubic centimeter has a resistance of millions of millions of ohms; but a glance at the accompanying table will show that paraffin may have more than 500 times the resistance of shellac! A coil wound with cotton-covered wire and dipped in paraffin is usually well insulated Sealing wax is good for fastening . wires in place and generally for use where large quantiies are not required. It remains firm and is easy to apply. It has a much higher resistance than beeswax or other ordinary waxes which the amateur is tempted to use. Where large quantities of wax-like material are required, sulphur can be used to advantage. It is much better than sealing wax, as the table indicates. It may be better than ordinary paraffin and is much more durable where it is exposed to wear. A beautiful panel can be made of wood and its possibilities are tempting to the no'vice. When he hears of the high prices quoted for patented panel material, he has another argument in favor of wood. If he does use wood, he should select the hardest available and give it a thorough painting with hot paraffin (hard wood like mahogany j best and is quite practical) ; but the best panels are made from rubber or some chemicallyprepared insulating material. Hard rubber is one of the best insulators known, being superior even to porcelain or glass. Mica or moulded mica can be used to advantage for insulating small parts. Climate may well be considered in addition these fundamental characteristics of in ulators, since both temperature and humidity change the resistance materially, and may make considerable difference in the operation of a station. A change of ten degrees in temperature may make a change of sixteen times the original resistance of a substance such as beeswax Sealing wax is one of the most stable insulators at normal temperatures, but all insulators are affected by heat. In government experiments it was found that bakelite had 300 times as much resistance at 25 per cent, humidity as at 90 per cent. Such substances as marble slate, and hard fibre, which are slightly porous' ai;eualso affected, but to a lesser degree Although shellac absorbs much moisture its resistance changes little with humidity. ' SURFACE LEAKAGE ONE of the chief causes for changes in the resistance of materials with humidity is surface leakage. This is an important consideration when choosing insulators for use in humid climates. The surface leakage is not caused