O. N. Rood on thermo-electric currents by percussion. 13 
ratus, obtain at will, a fall of 1, 2, 8, 4 or 5 inches successively, 
by raising the ball by the string, and using in turn each of the 
five holes in the brass plate. By this means the production of 
accidental thermo-electric currents from the heat of the hands is 
avoided, as the string and bent rod enable the observer to make 
the necessary adjustments from some distance 
alls on a thermo-electric couple #4, consisting of a 
ciptal wire of German silver and iron soldered together, or 
better, of a compound plate of the’ same metals, the juncture 
being soldered, as when plutes are used, the couple suffers but 
little i injury from the repeated falls of the ball. In the selection 
of these two metals for the couple, a suggestion of Poggendorff 
is followed, who showed that they give a strong current when 
their juncture is heated. The couple is so arranged that the 
ball strikes just on the juncture of the two metals, and there by 
means of the heat developed, produces a thermo: electric current. 
The juncture of the two metals was generally insulated by silk, 
¢., to prevent the heat from being immediately conducted o 
The two farther ends of the couple were fastened. by bos bind- 
ing screws ss, which were in metallic connection with a delicate 
galvanomete 
Below the eenegae is the brass anvil A. 
A certain amount of heat is developed at the junction of the 
couple by a given fall of the ball; if now the couple were left 
in contact with the ball and anvil after the fall, this heat would 
be rapidly conducted away; it therefore became e necessary to 
contrive, first, an apparatus for raising the ball instantly after 
its fall out of contact with the couple, and second, some arrange- 
ment for raising the couple at the same instant out of contact 
with the anvil. The ea of these ends is accomplished by 
the lever L, the shorter arm of which is cut out so that when it 
quickly pressed down, and fastened by turning the bent 4 
at w. The lever thus raises the ball 4 4 inch above the couple, 
and the latter itself acting at the same instant as a spring, 
raises itself by its own elasticity above the anvil. The wires 
from the binding screws were connected with an apparatus for 
breaking the circuif, in which small cups of mercury were 
This portion of the apparatus was placed on a table; -the galvan- 
ometer, however, on a shelf attached to the wall ‘of the room 
‘with brass nails, it being found that iron nails exercised a con- 
siderable effect on the astatic needle. When thus arranged, and 
observed with the perpen’, ii steadiness of the needa, was 
nat sensibly affected by a per ee anon the room. | 
a5 
