The motion picture projectionist (Nov 1931-Jan 1933)

November, 1931 Motion Picture Projectionist il The manufacturers of motor generators are now advocating a minimum output of 80 volts for low intensity arcs to obtain the proper ballast action from the resistance connected between the generator and the arc. It is their contention that anything less than 80 volts results in an unstable arc which is unsatisfactory for proper projection. For the Hl-Low and Hi-Intensity arc the line voltage must be considerably higher as in these lamps the current is higher. Therefore, the arc voltage also goes up and in order to get a steady arc, the line voltage must go up in proportion. Having considered some principles covering the use of resistance in arc circuits, an explanation of the prac Table B Data Covering Arc Ballast Rheostats for 100-115 Volt Circuits. Fig. 2 tical application in the form of resistors and rheostats may be useful. This explanation will cover the standard types in general use. Resistor Composition and Construction The resistance for use as ballast may be furnished as a fixed resistor or as a rheostat consisting of a number of resistors with means for adjusting the amount of resistance in the circuit. Resistors have been generally used for low current applications and rheostats for both low and high current arc circuits. The fixed resistors are supplied with a resistance element of nickelchromium ribbon formed into a Uchannel and mounted on a transite bar. This construction makes a light, compact resistance unit and the ribbon presents the maximum surface possible for heat radiation. The alloy is non-corrosive and eliminates difficulties due to oxidation. It has a very low temperature coefficient so that the change in resistance with change in operating temperature is not appreciable. It is capable of carrying severe overloads without injury. Extra terminals are provided for adjustment of current to suit the operating requirements. The resistance unit is supported on an angle iron frame and enclosed in expanded metal. The variable rheostats are made up using a suitable combination of resistors to furnish the specified capacity. In order to provide control > c B o c "* w C u t a = £ 0<U S S O bo . C C « So Dimensions Type of Arc Height Width Depth Reflector — Low Intensity 55 24-25 3 8 17" 6-%" 15-%" Reflector — Hi. Intensity 55 30-90 5 13 25" 16" 18-%" Condenser -Hi. Intensity 60 30-120 5 19 25" 20" 18-%" Condenser -Hi. Intensity 70 60-150 5 19 25" 24" 18-%" Condenser -Hi. Intensity 75 90-180 5 19 25" 28" 18-%" Condenser -Hi. Intensity 75 90-210 5 25 25" 32" 18-%" in 5 ampere steps, wire bar resistors are used for three steps of 5, 10 and 15 amperes. Wire bar resistors are made by winding an alloy resistance wire, having zero temperature coefficient, in the form of a long coil. The coiled wire is wrapped on a narrow transite support, eliminating the sagging of the wire and touching of adjacent coils. The additional resistors in the variable section and all the resistors in the fixed section of the rheostat are made of Ribohm units which are identical in size and shape. It is possible to furnish any desired capacity using Ribohm units of one size with one set of wire bar resistors. The resistor units are mounted on a transite faceplate as shown in Fig. 3. The units are spaced by insulators so that it is impossible for adjacent units to touch. The switches for adjusting the current in 5 ampere steps are also mounted on the faceplate. The rheostat, shown in Fig. 1, is furnished with 5 switches giving 11 steps of control. The present trend is to use knife switches for control due to the large contact surface available insuring perfect contact. All the resistor circuits are connected in parallel so that the current through the switches is limited to 30 amperes. The copper bus links, shown in Fig. 2, provide a means of adjusting the resistance of the rheostat to meet a wide range of line and arc voltage. By shunting the links across the resistor sections the voltage drop can be varied in three steps of 5 volts each. This feature is a real advantage as it enables the projectionist to adjust the voltage drop in the rheostat to suit different arc voltages and to insure the calibration of the rheostat. The faceplate supporting the switches and resistor units is supported in a substantial angle iron frame and mesh enclosure. The live parts are enclosed and provision made at the bottom in front of the faceplate for making connections. Large studs with terminal lugs are provided for making connections. The depth of the cages used on rheostats for Fig. 3 operation on a given voltage is the same. Table B gives the data and dimensions of rheostats for use on 100 and 115 volt circuits. (To be continued) E. R. P. I. Awarded Decision In Default Case Judge Clyde R. White of Minneapolis has handed down a decision awarding Electrical Research Products the full amount claimed with interest and costs, in its suit against the Peoples Theatre Company, operating the Paradise Theatre, Minneapolis. The suit was instituted because of defaulted payments on Western Electric reproducing equipment leased the theatre by Electrical Research Products. After payment was defaulted Electrical Research Products removed the equipment and brought suit to recover the amount in default. The Theatre Company set up a counterclaim for $6,100 alleging interference with its use of the equipment. Judge White's decision followed. In his decision Judge White said' in part: — "The defendant (Peoples Theatre Co.) could not terminate this contract except upon the option of plaintiff (Electrical Research Products) . . . the sealing of the machine would not be deemed a breach of the contract, such as would warrant its termination because it took place by the mutual agreement of the parties ... I am also clear that the contract in question is not a conditional sales contract."