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LII. The Measurement of Light. 

 By Norman Campbell, Sc./>., and Bernard P. Duddim; *. 



Summary. 



IT is maintained that in order to establish a scientifically 

 or legally satisfactory system of measuring any physical 

 magnitude it is not sufficient to define the units to be em- 

 ployed ; it is necessary also to state the laws of measurement 

 involved. Photometry provides an exceptionally favourable 

 illustration of this necessity ; the laws of measurement 

 underlying photometry are therefore considered in some 

 detail." 



1. International congresses have from time to time fixed 

 with great elaboration the units of certain practically impor- 

 tant physical magnitudes. The results of their labours are 

 embodied, not only in scientific treatises, but also in much 

 national legislation. It appears to be thought widely that, 

 when the units have been tixed, an entire system of measure- 

 ment has been established and that no further question can 

 arise concerning the value to be attributed to any magnitude. 

 We are of the contrary opinion. It appears to us, for example, 

 that when the unit of resistance is fixed, we know certainly 

 that one definite body has a resistance of 1 ohm ; but we do 

 not know certainly what other bodies have a resistance of 

 1 ohm or what bodies, if any, have a resistance of 2 ohms. 

 And knowledge on these matters, which are not decided, is 

 of much greater importance from every point of view than 

 knowledge of those which are decided. 



Let us put the matter practically. An Act of Parliament 

 has laid down what is one ampere and one volt, and decided that 

 one watt-second is the energy required to maintain one ampere 

 under a potential of one volt for one second. Well and good. 

 But when we are presented with our electric-light bills^we 

 are asked to pay for a good many kilowatt-hours. And the 

 Act has never told us what kind of current, under what kind 

 of potential, for what kind of time, is the precise number of 

 kilowatt-hours for which we are asked to pay. It appears 

 to us that the Courts ought to decide that we cannot be 

 legally forced to pay for anything but one watt-second, 

 because otherwise we are paying for something of which the 

 nature is wholly unknown to the law. 



Doubtless counsel would advise us that, if we tried to 



* Communicated by the Authors. 

 Phil Mag. S. 6. Vol. 44. No. 261. Sept. 1922. 2 P 



