12 O..N. Rood on thermo-electric currents by percussion. 
Art. Il.—On the production of Thermo-electric currents by percus- 
sion; by O. N. Roop, Prof. of Physics in Columbia College. 
_ THE production of thermo-electric currents by friction was 
observed by P. Erman in 1845,’ but I do not know that the sub- 
ject of the present article has ever been examined with any care. 
For the purpose of studying the thermo-electric currents pro- 
duced by percussion, the apparatus represented in the figure 
\\ 
DTM Sain 
Bi 
A 
free and falls. The rod is fastened at such a height that when 
its bent end is in the highest of the five holes, the distance be- 
tween the lower surface of the ball and the anvil below, is one 
inch. The holes in brass plate again are exactly one inch apart, 
so that the experimenter can easily, without altering the appa- 
* Arch. de EL, v,. 477; Inst, No. 614, p. 355. 
