International projectionist (Jan 1943-Dec 1944)

provides a wiping or self-cleaning type of contact. There is another type of toggle switch with a roller that closes the circuit by pressure against the contacts and which is widely used in radio sets. This type is not self-cleaning and has a shorter life. Silver-plated contacts are more satisfactory than unplated contacts of either copper or berylium copper. Still more satisfactory is a laminated material consisting of coin silver and berylium copper. The copper provides the necessary spring and, therefore, tension, and the silver furnishes the electrical conductivity. Such switches were available before the war on special order and it is to be hoped they will become standard after the war, unless, of course, a still better material is developed. Some toggle switches can be taken apart for inspection, adjustment and cleaning. Care must be taken, however, to disassemble them with due regard for the many small parts inside or they never will function again. It would be well to experiment with a spare switch first before going to work on one of the equipment switches. A very small amount of light grease may be applied to the contacts, but it should be borne in mind that grease always tends to pick up any dust and grit flying around in the air. So it is better not to apply the grease if the switch is exposed to such an atmosphere, on the basis that grit increases wear more rapidly than normal operation of the switch. Maybe some of you have had burned contacts on your sound system motor starting switch; in fact they have to be replaced all too frequently. If the switch is properly rated for the service, short life may be due to the practice of using the motor switch to run down the film after threading. This practice may result in opening and closing the switch several times before the motor is up to speed. If we remember that the starting current of a soundhead motor is of the order of 25 amperes and the running current probably not more than 5 amperes, it is easy to see that the switch is opening a circuit carrying much more than normal current. Under such cir cumstances the switch cannot be expected to stand up. While such switches are of the quick-acting type, a heavy arc will be drawn every time the switch is opened in the running down operation ; this heats and burns the contacts. Anyone who insists on continuing this practice must expect to have switch failures. He can prolong the life of his switch by frequent inspection and by keeping the contacts free from pitting and burn Watch Oxidation When toggle switches are used in plate and heater supply circuits the problem is somewhat different. In the first case we have a high voltage, low current circuit, and in the second case a low voltage, high current circuit. Poor contact in both cases will result in noise. There may be burning eventually and then failure. For this use the knife switch type of contacts is very necessary and, furthermore, silver-plated contacts or those made of the laminated material mentioned above are necessary. Non-conducting oxides form on most metals when they are exposed to air. These oxides increase the resistance of the circuit to the point where current will not flow unless there is sufficient power in the circuit to break down that oxide. Silver forms a conducting oxide and therefore is ideal for this purpose. Unplated contacts in plate circuits give more trouble than in heater circuits, but there is evidence of trouble from this cause in heater circuits. If unplated contacts are used in speech circuits, continual trouble will be experienced unless the contacts can be cleaned frequently. We can say that silver contacts are a "Must" in speech circuits. Rotary switches frequently are used in power circuits when sequence operation is required; for example, in amplifiers employing gas tubes that have to be heated before the plate is turned. Generally, standard switches can be used for this application as the power is sufficient. However, frequent inspection and cleaning of the contacts will prolong the life of the switch, if at the same time the contacts are kept adjusted so that there is adequate pressure. The contacts In recognition of the urgency for the continued salvage of every bit of copper, the National Carbon Company has designated on each package the number of ounces of copper that can be saved from the carbons enclosed therein, as shown below : 8mm x 14" 8ram x 12" 7mm x 14" 7mm x 12" 7mm x 9" 6mm x 9" 'National" "Suprex" Positive Carbons "National" "Orotip" C Negative Carbons 3.2 2.7 1.5 1.3 1.6 1.3 oz. copper drippings on most of the switches are of the knife switch type, which is most desirable, as previously mentioned. Different types of rotary switches are used in speech, and plate and heater supply circuits. Contacts should be of the self-cleaning type and definitely should be silver. Many switches of this type are used in the radio industry but, generally, these are not rugged enough to give satisfactory service in the projection room. This is a point to remember in obtaining replacement switches if you expect the switch to last. The better types of switches in this class require very little maintenance and have an exceptionally long life. They do not appear as rugged as many of the toggle switches, but that is deceiving as they will perform better. It seems to be a common belief that the higher the current rating of a switch the better its performance will be in any circuit. But a switch developed for power circuits will be apt to cause all kinds of trouble when used in a speech circuit. For one thing it probably will have unplated copper or berylium copper contacts. Also it may not have self-cleaning contacts. Switches that have been especially designed for speech circuits should be used in such circuits so as to avoid complex maintenance problems. Slide Switches Rugged Slide switches are in the same class as toggles and the same considerations apply. These switches have found their widest application in laboratory test instruments and it can be said that the standard slide switches generally are not rugged enough for the projection room. In case you have not encountered the slide switch, its action is the same as that of a toggle switch, but it is actuated by a button which slides along the panel on which the switch is mounted. By "key switch" we refer to the telephone type. These are used in many types of speech circuits and there are switches of this type on the market that will handle small amounts of power. The contacts are of the self-cleaning type, it being accomplished by what is known as "follow" of the springs. That is, the making contact continues to move slightly after actual contact is made, thus sliding on the contact which it has made. "Follow" also insures a positive contact that will not chatter and cause intermittent interruptions of the circuit. Many different contact materials are used, depending upon the application, so in selecting such a switch careful attention should be paid to the type of contacts used. These switches come in many spring combinations so that numerous switching combinations may be ob(Continued on page 18) INTERIVATIONAL P R O J E C T I O IV I S T