new Standard of Light, 401 



The resistance of the mercury rheostat can be easily altered 

 by moving the bridge b along, the two parallel mercury-grooves. 

 If the bridge is taken out, the total resistance of the rheostat 

 is in circuit. 



If the bridge b is close to the two terminals ff, the resist- 

 ance of the rheostat is nil. 



This range of resistance with about 6 to 10 volts will prove 

 sufficient to make the current strong enough and to keep it 

 constant for many hours, especially if the precaution be taken 

 to open the circuit when no light is required. The bridge b 

 consists of a strip of copper at least 2 centims. broad and 1 

 millim. thick. The knife-edges which dip into the mercury 

 are amalgamated. 



The current-measurer G has been gauged by comparison 

 with a standard tangent-galvanometer ; so that the currents 

 indicated by certain deflections of the needle are correctly 

 known in absolute measure. 



Whenever a photometric measurement is made, the current 

 is adjusted to its defined strength ; i. e. the given known de- 

 flection is procured by moving the copper bridge b. 



If the instrument G is well constructed, this adjustment of 

 current-strength can be executed as accurately as weight- 

 measurement by a chemical balance. 



Correction for the Standard. 



Although with the above arrangement it will be always 

 possible to keep the current constant and up to its defined 

 amount, it might nevertheless happen under particular cir- 

 cumstances that the current producing the light has been 

 rendered different from the current for which the standard has 

 been defined. 



In this case the following correction can be applied : — 



1 



(i+«)g) 2 -« 



where c is the current for which the intensity of the light has 

 been defined as unity, y the actually observed current, and u 

 the coefficient for platinum which gives the percentage varia- 

 tion of resistance at high temperature, 1500°-2000° F. for 

 1° Celsius. 



This correction has been developed on the supposition 

 that the light produced in the given piece of platinum is pro- 

 portional to the work done by the current through the resist- 

 ance of the platinum, and further, that, temperature and light 



