334 
Journal of Agricultural Research 
Vol. V, No. 8 
In the multiple-point switch the leads from the different groups of 
thermoelements terminate in a double row of studs arranged in segments 
of concentric circles, and the galvanometer leads terminate in two metal 
rings concentric with the studs (PI. XXXIII, fig. i). Metal strips, passing 
through a vertical shaft at the center of the circles, complete the circuit 
from studs to rings, the ends of the strips being bent to touch edgewise. 
On turning the shaft by means of the handle at the top, the strips are 
shifted from one pair of studs to another, thus connecting the different 
systems with the galvanometer. The switch includes studs not only 
for the thermoelement groups described above, but also for resistance 
thermometers described beyond, so that the same galvanometer will 
serve for several systems. 
The galvanometer with which the electromotive forces in the thermo¬ 
element circuits are detected is a reflecting instrument of the D’Arsonval 
type, with a coil resistance of 39 ohms. When critically damped, it has a 
period of 7 seconds, and a sensitivity such that an electromotive force 
of approximately 2 microvolts in either circuit will cause a deflection of 1 
mm. on a scale 1 meter from the mirror of the galvanometer. 
With this galvanometer the number of thermoelements in each circuit 
is sufficient to cause a fairly large deflection when the temperature of the 
zinc wall is only slightly different from that of the copper wall. In the 
bottom section, for example, there are 10 thermoelements, the smallest 
number in any section, each with four differential couples, and each couple 
having a thermal electromotive force of close to 40 microvolts per degree 
of temperature difference between the junction at one end and that at the 
other. All 40 couples being in series, there would be a total electromotive 
force of 1,600 microvolts for an average difference of 1 degree between 
the temperature of the copper wall and that of the zinc wall in this section, 
or 16 microvolts for an average difference of 0.01 degree. Since an 
electromotive force of about 2 microvolts will cause a deflection of 1 
mm., a difference of 0.01 degree would cause a deflection of at least 
7 mm. It is easy to read a deflection of less than 1 mm.; consequently 
the effect of a temperature difference as small as 0.001 degree between 
the junctions at opposite ends of the thermoelements in this may be 
observed. The effect of such a difference in the other sections would be 
greater, because of the larger number of elements; the 16 in the top 
would cause a deflection of more than a millimeter; the 27 in the upper 
zone of the sides about 2 mm.; and the 42 in the lower zone more than 
3 mm. 
Controlling the Temperature op the Zinc Wall op the Chamber 
The temperature of the zinc wall is raised or lowered by heating or cool¬ 
ing the air confined in the narrow space adjacent to the outer surface of the 
zinc, which has a corresponding effect on the wall. Parallel with the 
wall, and about 4 cm. outside of it, is a wall of cork board 38 mm. thick, 
