. Electronic apparatus for biological research. Electronic apparatus and appliances; Biology -- Research. LIGHT SOURCES AND DETECTORS to the surface area of the filament and this increases linearly with filament diameter; but the cross-sectional area increases as the square of the diameter and thus increases more rapidly than the rate of evaporation. Thus low voltage lamps are more efficient than high voltage lamps of equal wattage, as they can operate at a higher temperature without increased evaporation. E 100 o c o c E 50 10. Phofoflood 2-15 h Automobile lamps Gas-filled general service lam
. Electronic apparatus for biological research. Electronic apparatus and appliances; Biology -- Research. LIGHT SOURCES AND DETECTORS to the surface area of the filament and this increases linearly with filament diameter; but the cross-sectional area increases as the square of the diameter and thus increases more rapidly than the rate of evaporation. Thus low voltage lamps are more efficient than high voltage lamps of equal wattage, as they can operate at a higher temperature without increased evaporation. E 100 o c o c E 50 10. Phofoflood 2-15 h Automobile lamps Gas-filled general service lamps 1,000 h. Vacuum lamps ,r Carbon â / lamps II I I I I I I I I I I I I I I I I I I M. 2,A00 2300 Operating temp. Figure Working characteristics of various types of tungsten lamps: effect of filament temperature on efficiency and brightness {after Bourne'^) The life of a lamp varies approximately as the reciprocal of the fourteenth power of the voltage. Thus the life of a general service lamp would fall by some 13 per cent if its supply voltage were raised by 1 per cent. The effect of altering the supply voltage to such a lamp is shown in Figure Approxi- mate figures might be that for an increase of 1 per cent in voltage there would occur an increase of 0-5 per cent in current, 1-5 per cent in wattage, 3-6 per cent in light output and 2 per cent in light output per W. Several useful points arise from these figures. First, if a bright compact light source is required it might be convenient to use a relatively cheap lamp such as a 6V automobile headlamp and over-run it by 10 to 20 per cent of its rated voltage. This would give the brightest source available from tungsten (5,000 cd/cm") but with the disadvantage of frequent lamp changes. Lamp life could be slightly increased by bringing the filament slowly to its maximum temperature, perhaps by placing a suitable 'Brimistor'* in series. * A resistor possessing a large negative temperature coeffic
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