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344 RICHARDSON'S BLUEBOOK OF PROJECTION
reflector type lamps which use a maximum current of 28 to 42 amperes. (33) The current range and combinations are as follows :
28 to 32 amperes, 12 mm positive, 8 mm negative. 32 to 42 amperes, 13 mm positive, 8 mm negative.
(34) At the present time this covers the range of amperage in general use. There are positives made in 10, 12, 13 and 14 mm diameters having a current range of 21 to 24; 28 to 32 ; 32 to 42 and 42 to 52 respectively. (35) These four sizes may be expected to produce an unsteady arc if current up to or in excess of 25, 34, 44 and 55 amperes respectively is forced through them.
(36) Forcing these carbons above their maximum capacity adds but very little to the illumination of the screen. On the other hand, burning them below maximum capacity will produce the same effect — an unsteady arc and unsatisfactory screen illumination.
(37) Approximate candle power may be calculated for different amperages. Taking the 12 mm combination as described above, at 28, 31 and 34 amperes the crater floor will have the following square mm areas : 49, 53.5 and 60. The 13 mm carbon crater floor will, at 34, 40 and 44 amperes, have square mm areas of 61, 72.4 and 77.9.
The crater brilliancy per square mm of these carbons is given as 130 to 175 candle power. A fair average for d. c. arcs is 165 c. p., so if we multiply the square mm area of the crater floor by 165, we shall have a very close approximation of the total candle power, though not all of it will be available to the collector element (mirror) under even the best conditions. The above c. p. is for d. c. arcs only. The average a. c. is 130 c. p. per square mm.
(38) Within their current range SRA carbons are capable of giving from about 5,800 to 6,880 candle power for the 10 mm trim; about 6,750 to 9,950 candle power for the 12 mm trim; about 10,200 to 12,000 for the 13 mm trim and about 14,250 to a trifle more than 15,000 candle power for the 14 mm trim.