. Electronic apparatus for biological research . 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
. Electronic apparatus for biological research . 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 coefficient. Its resistance is initially high, but falls rapidly as the Brimistor warms up. ED. 340
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