590 



HISTORY OF SCIENCE. 



sufficiently obvious. A discovery was made in 1821 by Seebeck of 

 Berlin, contributing another link to the bonds which connect electri- 

 city with other forces. Let A, Fig. 299, 

 be a bar of antimony, and B a bar of bis- 

 muth, the metals being in contact at h 

 and c. When heat is applied at //, or, to 

 state the fact more generally, so long as 

 the Junction h is warmer than the junc- 

 tion c, a current of electricity will circu- 

 late in the metals in the direction shown by the arrows. A mag- 

 netic needle poised on a pivot within the circuits would indicate the 

 current by its deflection. This was Seebeck's discovery. Not anti- 

 mony and bismuth only, but any two metals whatever, will in their 

 own degree produce the same effects. Later researches have shown 

 that the transference of heat between any heterogeneous conductors 

 suffices to set electricity in motion. Fig. 300 shows a galvanometer 



FIG. 299. 



FIG. 300. 



at A ; s is a silver and P a platinum wire in contact at -f ; it suffices to 

 warm the junction of the wires by a spirit-lamp to obtain a strong 

 deflection of the needle. If, instead of the silver and platinum wires 

 between the terminals of the galvanometer, we make use of a single 

 copper wire coiled or tied in a knot at +, we shall obtain a current 

 on heating the wire at one side of the knot, and the reverse current 

 on heating it on the other. Seebeck's discovery of thermo-electricity, 

 besides its theoretical interest, had the effect of giving to science a 

 delicate thermoscope, which in this work has been already described 

 in connection with radiant heat (page 507). 



Electricity has arrived at the stage in which its laws resolve them- 

 selves into the statement of accurately-defined quantitative relations. 

 The electrician must have, therefore, instruments for accurate measure- 

 ment. These have not been wanting. An instrument far more con- 

 venient than the torsion balance (page 335) for measuring charges of 

 static electricity is Sir W. Thomson's " attracted-disc " electrometer. 

 Other instruments contrived by the same distinguished electrician will 

 be found described in the books. The "mirror galvanometer" repre- 

 sented in Fig. 301 is a good example of the delicacy of modern electric 



