junctions constituting a thermo-electric pile. I shall connect this pile with our galvanometer, and you see it is so sensitive that, if I hold my hand near it, the heat radiating from my hand at once causes a movement of the galvanometer needle and a corresponding movement of the spot of light on the scale. When I heat the other set of junctions, you observe the movement of
Fig. 57—Small thermal pile. Observe the junctions of bismuth and antimony. The distance from a to b is one-fourth of an inch.
the spot of light is in the opposite direction, because the current passes in the opposite direction through the coils of the galvanometer.
Let us now examine the muscle by a thermo-electric arrangement. I have two very small thermal piles, and I connect them together, so that if I heat one by bringing my hand near it, the spot of light from the gal-
