92 - L. Sechwendler—On a new Standard of Light. [No. 2, 
If the bridge 6 is close to the two terminals f f, the resistance 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 is taken to open the circuit when no light is 
required. The bridge 6 consists of a strip of copper at least 2 em. broad 
and 1 mm. 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 correstly known in absolute measure. 
Whenever a Photometric measurement is made the current is adjusted 
to its defined strength, 7. e., the given known deflection 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 chemi- 
cal 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 neverthe- 
less happen under particular circumstances 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 
‘= ————_; 
(1 + a) =) —a 
Y 
where ¢ is the current for which the intensity of the light has been 
defined as unity. 
y the actually observed current, and a the co-efficient for platinum 
which gives the percentage variation of resistance for high temp, 1500°- 
2000° F. for 1° Celsius. 
This correction has been developed under the supposition that the 
light produced in the given piece of platinum is proportional to the work 
done by the current through the resistance of the platinum, and that, 
further, temperature and light are proportional. These suppositions are 
almost correct for small variations of the current. 
