International projectionist (Jan 1963-June 1965)

change is made to more powerful lamps. Undersize wires warm up when carrying the 40 to 180 amps, burned in an arc lamp. This is dangerous, particularly when the heat is confined by insulation, cable sheathing, conduits, etc. The temperature may rise high enough to char the insulation and render the transmission wiring more unsafe than ever. A corroded or loose connection anywhere in an arclamp circuit will cause serious operating difficulties even when the transmission wires from the rectifier or generator to the arc lamp have adequate current-carrying capacity. And the heavier the arc current, the more likely that a bad connection will offer erratic resistance to the current and cause the arcs to behave unpredictably. Thus the projectionist is never allowed to forget that his projection lamps are the most sensitive of electrical devices. They brook no deviations whatever from their normal current requirements. Are the Carbons at Fault? Time and again the carbons get unfairly blamed for electrical defects in the arc-lamp circuits. If the carbons escape the projectionist's wrath, the current source or the lamps, themselves, may be the targets of a few choice cuss-words. And while it is true that carbons, generators, rectifiers, and lamp mechanisms can be at fault, a misbehaving arc is usually only reacting to current variations or inadequacies caused by a bad connection at the current source, the ballast rheostat, the fuse blocks, or in the lamp, itself. The heat of the arc is very bad for the wiring inside FOR CURRENTS FROM 75 TO 90 AMPERES 90 TO 110 AMPERES 110 TO 135 AMPERES FOR RUNS UNDER 15 FEET USE WIRE SIZE • 3RH • 2 RH " I RH FOR RUNS OVER 15 FEET USE WIRE SIZE Courtesy Strong Electric Corp. FIG. 2 — A current-controlling ballast rheostat and a lamphouse table switch are absolutely necessary in each arc -lamp circuit powered by a motor-generator set. Rectifiers, on the other hand, require no heat-producing, current-wasting ballast. Many rectifier manufacturers also recommend elimination of the usual table switch, the current being turned on and off at the rectifier, either directly or by means of a relay button at the lamp. International Projectionist March 1963 Binding post tightened / for better contact ndin^post lu£ FIG. 1 — An electrical binding post should be tightened securely in order to insure good electrical conductance. Note how the contact area of a round wire (shown in cross-section) is gready increased by squeezing it out of shape when the binding post nut is tightened. A sufficiendy large contact area is best established by use of a washer-shaped binding-post lug. Insufficient contact may offer enough resistance to the flow of current to develop heating and "burning" of the connection. This is especially true of arclamp circuits because of the heavy currents involved. the lamphouse. The flexible stranded-wire leads are especially liable to deterioration. The individual copper strands are rather fine, and consequently expose a large total surface to the oxidizing influence of the air. Badly oxidized wires offer increased resistance to the flow of current, and hence hasten their own destruction by heating up. Flexible wires will last for many years if large enough to carry the current without heating, but may in some cases deteriorate rapidly in a hot, poorly ventilated lamphouse. (Another good reason to heed the manufacturer's advice on lamphouse ventilation ! ) Examine the arc leads in each lamphouse to make sure that they are stiff and springy. If the individual strands are dark brown in color, and seem to be brittle and crumbly when bent sharply, the wiring should be replaced. The carbon-holder or feed-head binding posts should be examined several times a year for evidence of looseness or corrosion. Lugs and other contact surfaces should be polished with extra-fine, or No. 00 sandpaper, and the binding-post nuts brought up tight when replaced. A loose contact "burns" and fails to transmit the power properly. Never use emery paper or cloth for polishing or cleaning electrical contacts. Emery dust conducts current to a slight extent. Sandpaper is safe because quartz sand ( unless molten ) is a non-conductor of electricity. Inasmuch as the same number of amperes flow in all parts of a circuit, the connections should also be clean and tight at the generator or rectifier terminals, the ballast rheostat, and at all fuse blocks and switches. In cases where the binding posts are so badly corroded that the lug nuts cannot be loosened without damage, a few drops of kerosene and thin lubricating oil applied to the binding post and left on for several hours often works miracles. The carbon-holding jaws of simplified HI lamps and the contacts of rotating-positive HI lamps should normally never be filed or sandpapered. Filing roughens them and may get them "out of true" or spoil the fit. Roughened metal, by the way, oxidizes more rapidly than smooth, highly polished metal. Carbon holders are usually made of special heatresistant bronzes; and certain high-powered lamps have water-cooled contacts of pure silver, which is the best conductor of electricity known. The dull brownish coloration produced by a microscopically thin film of copper oxide (or silver sulfide) on the surface of the metal need cause no concern, and should not be removed. It offers no measurable resistance to the passage of current; and its formation is a normal occurrence. The copper commutator bars of motors and generators are 5.