ON THERMO-ELECTRIC CURRENTS IN CIRCUITS OF ONE METAL. 173 
-“T hope, then, a result of the Committee’s action may be to carry out an 
attempt of this kind for every class of variations for which the data give even 
the narrowest foundation. It might be applied, I believe, with success, as 
regards the main conclusion, to every case in which each of the three compo- 
nents has been well determined for even only THREE stations widely apart 
from one another. 
«Tt seems probable that an individual deflection of a magnetic storm cannot 
be identified in localities at very great distances from one another. This must 
certainly be the case if an individual deflection, and individual flash or flicker 
of aurora, are simply related to one another, because the individual auroras 
are certainly local in the sense of being only seen at once over a very limited 
area of the earth, being in fact actually situated at some distance of not more 
than 150 miles (which I believe is the highest estimate) from the surface. 
Hence it is probable that it will be found whether the seat of the disturbing 
action, producing an individual deflection in a magnetic storm, is above or 
below the surface, by comparing observations made at stations within a few 
hundred miles of one another, and endeavouring to identify a single disturb- 
ance in the three components at all the localities. If the three components 
could thus be determined at three localities so wide apart as to show con- 
siderable differences in the amounts, but yet not so wide as to render the 
identification of the disturbance difficult, the question whether the seat of the 
disturbance is in the earth or the air would be answered with high proba- 
bility. 
“JT remain, yours very truly, 
(Signed) ‘© WititAm THomson.” 
On Thermo-electric Currents in Circuits of one Metal. 
By Fuizrmine Jenxin, Esq. 
Lasr year I had the honour of directing the attention of the Association to 
the fact, that an electric current of considerable intensity may be obtained in 
a circuit of one metal by the application of heat to one or the other side of an 
interruption in the wire composing the circuit. The experiment is most 
simply performed by looping together the two ends of two perfectly similar 
wires connected to the terminals of a galvanometer, and heating one of the 
loops to a white or red heat in a spirit-lamp, or Bunsen’s burner. If the one 
loop rests very lightly on the other a current will be obtained, which in the 
copper wires will flow from the hot to the cold loop across the joint with 
sufficient intensity to deflect a moderately sensitive galyanometer, even with 
a resistance in circuit equal to 1000 miles of No. 16 copper wire. 
The electromotive force of the combination is about one-tenth that of a 
Daniell’s cell. With two iron loops a permanent current in the opposite 
direction is obtained, flowing from cold to hot across the joint, but the elec- 
tromotive force in this case is very much smaller. 
When the loops are drawn tightly together the current ceases, but reappears 
as soon as the strain is slackened. 
_ I was at the time unable to show the connexion between these singular 
currents and other electrical phenomena, but I am now, in consequence of 
further experiments undertaken for the Association, able to point out that 
connexion. 
