182 PHYSICS. 
d. Thermo-Electricity. 
Those electric currents are called thermo-electric which are produced by 
heat, as discovered by Seebeck. If two metal rods are soldered together in 
two places, so as to form a closed circuit, and the two places of junction 
have different temperatures, an electrical current arises, and is indicated by 
the deflection of the needle. In pl. 22, fig. 11, let ss’ be a small bar of bismuth, 
ses’ a bent strip of copper soldered to the extremities of the first bar at s and 
s'; also let ab be a magnetic needle playing on a pivot. At the beginning, 
when both joints have the temperature of the atmosphere, place the appa- 
ratus so that the plane of the rectangle, scs’, may fall in that of the magnetic 
meridian: the needle will then be parallel to the edges of the bismuth bar. 
On heating or cooling one of the two joints, the needle will immediately be 
deflected to one side or the other. Frequently an elongated rectangle of 
bismuth and antimony is employed, one of the joints being heated over a 
spirit lamp, and one of the longer sides of the rectangle held over a magnetic 
needle. Simple thermo-electric circuits have sometimes the construction 
of fig. 12, pl. 22. where ab is a bar of antimony or bismuth, and abcd a 
copper wire soldered to it; after heating, one of the joints is held over the 
needle. The action produced by different pairs of metal is very various. 
Antimony and bismuth give the most marked results ; all metals, however, 
form a series so constituted that when two of them are formed ito a 
thermo-electric circuit, and heated at one of the joints, the positive current 
passes at this place from the metal lower in the scale to the higher. This 
series is as follows: antimony, iron, zine, gold, copper, lead, tin, silver, 
platinum, bismuth. The further apart two metals are in this series, the more 
active is the current they produce. 
The most important laws of thermo-electricity are the following: 1. The 
quantity of the current electricity is the same in all parts of the circuit: 
2. The strength of the current is as the thickness of the closing wire, and 
inversely as its length. To prove this latter proposition we make use of a 
differential galvanometer ( pl. 22, fig. 28). This is distinguished from the 
ordinary galvanometer in consisting of two coils of equal length, thickness, 
and conducting capacity. Both wires are wrapped on the same frame. On 
allowing currents of equal strength to traverse the coils, but in opposite 
directions, no deflection of the needle will result. In this way we can 
convince ourselves of the perfect equality of two thermo-electric elements. 
To determine the conducting power of different metals, we make use of a 
very sensitive differential galvanometer, and the two thermo-electric 
elements represented in fig. 29. In the figure, ab and cd are two cylinders 
of bismuth, e the differential galvanometer, f a graduated ruler of from seven 
to ten feet in length, g a platinum wire stretched over this, and h a wire of 
that metal whose conducting capacity is to be compared with that of the 
platinum. The arrangements are such that the two currents pass through 
the galvanometer in opposite directions; both circuits, even to the wires g 
and h, are perfectly equal. The platinum wire can be shortened at pleasure 
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