Carbon filament lamps as photometric standards

The primary standards employed in physical measurements are of two kinds; (1) those which can be described in such terms that they can be accurately verified or reproduced from their specifications, and (2) those which are more or less arbitrary, and which cannot be accurately reproduced except by copying other standards of the same kind. The international ohm is a standard of the first kind, as it is specified in terms of the resistance of a definite column of mercury at a certain temperature, and it can be reproduced without reference to any other standard of resistance. The meter was originally intended to be such a standard, being defined in terms of the dimensions of the earth. But when it was found that the dimensions of the earth were different from what had been supposed and that the meter would require a new definition, the reference to the earth was abandoned and the meter became a standard of the second kind, only to be reproduced by reference to other meter bars, of which there were a sufficient number in existence to make it possible to maintain the meter indefinitely in this way. More recently the meter has been expressed in terms of the wave length of light so exactly that it could be reproduced accurately if all length standards were lost. Hence the meter has again become a primary standard of the first kind. However, meter bars are so permanent that in practice they are verified and reproduced by comparing with one another, without reference to the absolute specification in terms of the wave length of light.