COiS-CEElSrED IN PEODUCINa THE GEEATEE MAGNETIC DISTUEBANCES. 637 
Note on the Electromotive Force induced in the Eartlis Crust hy Variations of Terrestrial 
Magnetism. By Prof. W. Thomson, A.M., L.L.B., F.R.S. 
The evidence from observation adduced in the preceding paper tending to show that 
some “earth-currents” which have been actually observed have been the electro-mag- 
netically induced etfects of variations of terrestrial magnetism, appears to be a very 
important contribution to the discovery of the complete theory of these most interesting 
and perplexing phenomena. It necessarily, however, suggests the question. Is the elec- 
tromotive force induced by variations of terrestrial magnetism comparable in amount 
with that which is found in observations on earth-currents I There is scarcely occasion 
at present for working out a complete mathematical theory of the currents induced in 
the earth’s crust by any fully specified magnetic variations. This can easily be done as 
soon as observation supplies data enough to make it useful. In the mean time a very 
rough theoretical estimate of the absolute amount of electromotive force induced by 
such magnetic variations as we know to exist, is sufficient to render it certain that this 
electro-magnetic induction does very sensibly influence the observed phenomena of earth- 
cmTents. For if there be, as we know there is every day, a gradual variation of terres- 
trial magnetism over a large portion of the earth’s surface, amounting to a minute of 
angle in the direction of the dipping-needle, or to some thousandths, or not much less 
than one thousandth of force during several hours, this change of magnetism must 
induce in a length of a few hundred miles in the earth’s crust an electromotive 
force, which we readily see must be comparable with that which would be induced in a 
linear conductor of the same length if carried across the lines of magnetic force at the 
rate of a minute of arc (or a nautical mile) in several hours. In two articles communi- 
cated eleven years ago to the ‘ Philosophical Magazine ’ I explained the principles on 
which such an electromotive force as this is to be compared with familiar standards, as, 
for instance, that of an element of Daniell’s battery. Thus a horizontal conductor, 
1.400.000 feet long (or about 270 British statute miles), carried at the rate of 600 feet 
(or about one-tenth of a minute of arc) per hour in a horizontal direction perpendicular 
to its own length across the British Isles or neighbouring Atlantic ocean (where the' 
vertical magnetic force averages about 10 British absolute units of magnetic force), would 
experience an induced electromotive force amounting to sVod ' X 10 X 1,400,000, or about 
2.300.000 British absolute units of electromotive force. But, as I showed in the second 
of those articles, the electromotive force of a single cell of Daniell’s is about 2,500,000 
British absolute units. Hence the induced electro-magnetic force in question is about 
equal to that of a single cell. Some such electromotive force as this, therefore, must 
be induced in a length of 270 miles of the earth’s crust, by the ordinary diurnal varia- 
tions of terrestrial magnetism ; and the much more rapid variations in magnetic storms 
* See 1851, Second half year. “ On the Mechanical Theory of Electrolysis,” and “Applications of the 
Principle of Mechanical Effect to the Measurement of Electromotive Eorces, and of Galvanic Eesistances, 
in Absolute Units.” 
