High Temperatures by the Method of Colour Identity. 47 



were, therefore, as large as is usually met with in practice. 

 The motor headlight rilament was for 16 volts and 50 watts, 

 and therefore represented an extreme case. 



It will be seen from this that unless a very thick, short 

 filament be taken with abnormal end cooling, the measured 

 colour identity temperature will be that of the central bright 

 portion of the filament, within about 1 per cent., whilst in 

 the extreme case of the headlight lamp it is of the order of 

 2 per cent. It is obvious that the cooling effect for carbon 

 filament lamps is considerably less than for tungsten, and is, 

 in fact, quite inappreciable. 



It is thus clear that the colour-identity method gives 

 results which depend very closely on the temperature of the 

 central portion of the filament. If it may be assumed that 

 the method also gives the true temperature of lamp filaments, 

 the figures in Table I. and fig. 3 indicate the appreciable 

 difference of efficiency existing between the carbon and 

 tungsten lamps for the same temperature, and therefore 

 establish the selectivity of the tungsten filament in favour of 

 the shorter wave-lengths, a subject upon which much has 

 been written, and which has been thoroughly investigated bv 

 Dr. E. P. Hyde*. This difference in efficiency would, if 

 anything, be very slightly increased by taking into account 

 the end cooling of the filaments, the tendency of which is to 

 act in favour of the carbon lamp. Also, if the carbon fila- 

 ment is "greyer" than the tungsten filament in the visible 

 region the apparent difference of efficiency will be increased. 



(c) Referring to the curves shown in fig. 3, and bearing 

 in mind what has been said in the foregoing remarks, it 

 becomes of interest to know if a relation connecting lumens 

 per watt and temperature can be deduced from our knowledge 

 of the phenomena involved, and especially to ascertain how 

 nearly the experimental observations conform to such a 

 relation deduced from theoretical considerations. 



We have to consider, therefore, how the rate of dissipation 

 of energy by a lamp filament, i. e. the watts t, increases with 

 a rise in temperature, and also how the eye estimates the rate 

 at which this energy is radiated. 



The eye is only sensitive to a small portion of this energy, 

 i. e. that emitted in wave-lengths lying approximately between 

 0'3 fjb and 0'8 /x. Further, the eye does not appreciate the 

 intensity of the energy radiation in any wave-length over 



* See Hyde, loc. at. 



t The rate at which energy is radiated is power, and is spoken of 

 hereafter as radiant power. 



