cold, constituted a formidable loss. The electromotive force of each
thermo-electric element did not exceed 0.036 of a volt, and 1,800
elements were therefore necessary to work an incandescence lamp.

A most useful application of the thermo-electric battery for measuring
radiant heat, the thermo pile, was exhibited. By means of an ingenious
modification of the electrical pyrometer, named the bolometer, valuable
researches in measuring solar radiations had been made by Professor
Langley.

Faraday's great discovery of magneto-induction was next noticed, and the
original instrument by which he had elicited the first electric spark
before the members of the Royal Institution in 1831, was shown in
operation. It was proved that although the individual current produced
by magnetoinduction was exceedingly small and momentary in action, it
was capable of unlimited multiplication by mechanical arrangements of a
simple kind, and that by such multiplication the powerful effects of the
dynamo machine of the present day were built up. One of the means for
accomplishing such multiplication was the Siemens armature of 1856.
Another step of importance was that involved in the Pacinotti ring,
known in its practical application as the machine of Gramme. A third
step, that of the self exciting principle, was first communicated by Dr.
Werner Siemens to the Berlin Academy, on the 17th of January, 1867, and
by the lecturer to the Royal Society, on the 4th of the following
month. This was read on the 14th of February, when the late Sir Charles
Wheatstone also brought forward a paper embodying the same principle.
The lecturer's machine, which was then exhibited, and which might be
looked upon as the first of its kind, was shown in operation; it had
done useful work for many years as a means of exciting steel magnets.
A suggestion contained in Sir Charles Wheatstone's paper, that "a very