14. O. N. Rood on thermo-electric currents by percussion. 
The upper needle of the galvanometer was provided with a 
very fine glass rod, which served as an index, the breadth of the 
rod being only half of that of the divisions on the galvanome- 
ter circle. The end of the glass rod was blackened to render it 
plainly visible. Directly over the needle, a mirror silvered by 
Liebig’s process was placed at an angle of 45°; the index was 
observed with aid of this mirror and a small telescope magnify- 
ing five diameters; in this manner ,;';° could be est d 
The falling apparatus was enclosed by wooden poviana also 
the apparatus for breaking the circuit and the galvanometer. 
If these precautions are neglected accidental currents are con- 
stantly ose ale in the wires employed, and no reliable results 
can ned. It is farther necessary after exchanging the 
couple or Gendiag the binding screws, to allow the apparatus 
to remain at rest for two or three hours, so that the currents 
mperature o 
constant. I may remark, finally, that in eet of all these pre- 
cautious it is rarely the case that ay feeble and nearly constant 
accidental currents are wholly a 
The galvanometer was made by Duboseq; after balancing the 
magnetism of the needles it was found that the copper wire of 
the coil was so magnetic that the needles took up a position 30°— 
85° on either side of the zero point. I re-wound the frame 
with American wire, when the needle readily returned to the 
true zero; upon, however, bringing the two needles very nearly 
into the same plane, and carrying forward their astasie, the same 
difficulty was again experienced, when another sample of Amer- 
ican wire was tried with a result which was but little better. 
All of these samples when tested in the apparatus used for 
experiments on diamagnetism, were evidently magnetic, the 
French sample being strongly so. The difficulty was “evaded by 
bending the needles slightly out of the true plane, when they 
took up a position nearly east and west, and returned with cer- 
tainty to the true zero. In this state of inferior sensitiveness 
one simple oscillation consumed 18 seconds. There were suf: 
ficient indications to show that owing to the magnetism of the 
coil the needle was more sensitive to currents sam seem oe at 
10°-15° than when at 0°; it accordingly becam ry to 
calibrate the instrument with care. This was rer by one of 
the methods described by Melloni and quoted by Tyndall, (Heat 
considered as a mode of motion, p. 870). 
For degrees under 10° the constant currents employed in the’ 
calibration were produced by a small thermo-electric pile with 
one of its faces turned toward the exterior colder wall of the 
room, while the other face was directed toward an interior 
