and Surfaces of Incandescence Lamps. 381 
No. 1, the following values were obtained, connecting current 
and candle- -power :— 
Candle-power, 6 /ic Ourrent in 6/, 
K. VE amperes, a. Kae 
15874 "7582 2-09 
15024 “7065 212 
14215 ‘6720 211 
1:2988 6208 2:09 
1°1872 ‘HB59 2:03 
1°1225 ‘DD14 2,02 
The table shows that, with considerable accuracy, over a 
range from two to sixteen cz undles, the incandescence varies as 
the sixth power of the current. 
In those characteristics into which candle-power enters in 
any way, there is a considerable difficulty in getting results 
with sufficient accuracy to ee the constants of the 
characteristic equations. There is, bowever, one pair of 
variables, namely electromotive aay and resistance, both of 
which can be measured with very high accuracy over a great 
range; and some very interesting examples of these pressure- 
resistance curves are given by Professor Jamieson in his 
memoir above alluded to. 
A very short examination of the way in which an incan- 
descence lamp behaves under increasing electromotive force, 
shows that the resistance decreases with increase of electro- 
motive force, but that it does not decrease without limit; it 
tends to a minimum value, beyond which it appears to be 
constant. This is very strikingly shown for some of the Swan 
lamps tested by Professor Jamieson. In his paper certain 
formule are given connecting various lamp-constants, and as 
a first approximation to a pressure-resistance equation is given 
the following :— log P=log E+ar, 
where P is a constant, H= 1.M.I’., and r=resistance. 
It is not possible that such a formula should represent 
correctly the relation of resistance to pressure at high pres- 
sures, because it in no way expresses the fact that resistance 
tends to a minimum with increasing electromotive force. 
An empirical formula can, however, be obtained which will 
express this in the following way :— 
Let R be the resistance of a lamp measured with any elec- 
tromotive force, E, at the terminals. Let Ey represent the 
electromotive force at which the lamp just becomes incandes- 
cent, and let Ry be the corresponding resistance. Let 1 be 
the minimum resistance to which the lamp approximates, as 
Phil. May. 8. 5. Vol. 19. No. 120. May 1885. 2D 
