851 
2. The strength of current of the lamp may fluctuate; it must, 
therefore, be continually controlled. Care has been taken that the 
error in consequence of the fluctuations in the intensity of the lamp 
remains at the most of the order of 1°/,. 
As after a lamp has been lighted, the stationary and maximum 
intensity is not reached until some time after, the observations are 
always started half an hour after the lighting of the lamp, and the 
lamp remains burning all through the series of observations. 
3. When we allow light to fall on the thermopile Zh), the 
galvanometer Gr reaches its position of equilibrium within 2 seconds. 
When the radiation is continued, we see the deflection, after having 
been constant for a moment, slowly diminish. This is a consequence 
of the getting warm of the surroundings of those places of contact 
in the thermopile that are not directly irradiated. *) 
In order to minimize the influence of this error, the successive 
manipulations in the measurements are performed with constant 
intervals. , 
These intervals are marked by an electric clock, which every ten 
seconds closes a mercury contact for a moment. This brings about 
that via a relay through the key Wp, which can turn round a 
horizontal axis at #, the circuit is closed and broken at C and D 
alternately for ten seconds. Only during the 10 seconds that the 
shunt St is cut out at C, and the accumulator current put in at D, 
is the valve Ap open, and is the light admitted on the thermopile. 
It will be at once clear that in these ten seconds the resistance 
box BK cannot be adjusted so that the thermo-current is totally 
compensated. Therefore it is ascertained through a preliminary in- 
vestigation what value of the resistance is about required for every 
blackening to compensate the thermo-current. The galvanometer shows, 
therefore, a small deviation. As deviation is taken the difference 
between the unshunted zero position (1), i.e. when valve Kp is 
shut, key Wp is in position 1 and key S/ is out of the circuit, and 
the unshunted state of equilibrium (3), i.e. when valve Kp is open, 
key Wp in position 1 and key Sl is in the circuit *). 
1) The thermopile is provided with a cylindrical tube in order to prevent 
obliquely incident light. 
?) This phenomenon is met with to a much smaller degree in the later 
improved construction of the thermopile. 
5) As has already been said there is every time a period of ten seconds; 
this time is amply sufficient to allow the galvanometer with or without thermo- 
pile to resume its state of equilibrium, and to read the position reached on 
the graduated scale down to 0.1 m.m. 
