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O. N. Rood on thermo-electric currents by percussion. 15 
These, as it were natural sources of heat gave very constant 
currents, and by partially closing one of the caps of the pile, 
any desired deviation between 0° and 10° could readily be ob- 
taine 
It was found that for about 6° the deviation of the needle 
was directly pit tape to the strength of the current; for de- 
grees beyond this, it was necessary to construct a curve em ye 
ing the wher obtained experimentally. The ratio between 
the first and final deviation up to 30° was also obtained ; it was 
constant for 6°. These latter determinations were important, 
as after the first deviation the needle, owing to conduction in 
the couple, slowly sinks to 0°, and only then comes to rest. I 
was not able to measure with exactitude the time required for 
currents produced by falls of the ball from different distances to 
subside, the imperfect results obtained showed that it varied be- 
tween ik minutes up to 84 minutes, according to the distance 
fallen by the ball. 1t having been found then in the calibration 
experiments, that the force of the current was proportional to 
the deviation up to 6°, and farther, that the first deviation was 
proportional to the final deviation for the same number of de- 
grees, in the results given below, where the first deviation was 
peor eS the observations actually obtained and unreduced will 
n, but where the first deviation exceeded 6° the reduced 
ate will be found. 
As the total amount of heat produced by the fall of a body 
is divided between the falling body and that arresting its motion, 
it is evident that if the mass of the latter be small compared 
with that of the falling body, its temperature will, owing to this 
fact, be correspondingly high; and if the siete body be a 
thermo-electric element of small mass, a proportionately large 
deviation of the galvanometer needle will be produced. Te 
however, the couple at the moment of the percussion and after- 
wards, be allowed to be in metalli@ contact with the metallic 
ball, the temperature of the couple will by conduction be rapidly 
reduced to that of the metallic ball, so that the deviation of the 
needle will be very small, and the phenomena complicated. T 
illustrate this I give, in table 1, the small and irregular devia- 
tions which were produced v under these circumstances 
couple, and anvil all remaining in metallic contact after the fall. 
Taste 1. 
Distance fallen, lin. 2 in. 3 in, 4 in. 5 in. 
or? a is ° ; ° s° 
Deviations, “4 is te ie to ve 
ie to 
In table 1, a newly etint compound plate similar to that 
used in table 4 was employed. 
