42 
of one current on another and on itself, and succeeded in giving a theory 
of induction which was in accordance with the experimental laws. The 
laws were afterward experimentally verified by Weber. In 1849 the ex- 
periments of Kirchhoff on the absolute value of the current induced in 
circuit by another, and in the same year Edmund’s experiments on 
self and mutual induction, are important. In 1851 Helmholtz gave a 
mathematical theory of this part of the subject which he supplemented 
with an experimental verification. 
One of the most important of the series of experiments made by Henry 
was on the oscillatory character of the discharge from a Leyden jar. This 
he discovered from the effect of the discharge on a steel needle surrounded 
by a coil through which the current was made to pass. The results of 
these experiments were communicated to the A. A. A. S. in 1850, but he 
knew of the effect much earlier, certainly in 1842. Previously the anom- 
alous behavior of the discharge of a jar when used to magnetize steel 
needles had been noticed, but was attributed, I believe, to some peculiarity 
of the steel. Henry was the first to appreciate the true reason, although 
he could hardly at that time be expected to see the great importance of his 
discovery. 
Helmholtz in 1847 suggests that the discharge of Leyden jars may be 
of the nature of a backward and forward movement. There is a curious 
parallelism in the work of several investigators about this time, and .par- 
ticularly in that of Helmholtz and Thomson. In the Philosophical Maga- 
zine for 1855 there is a paper by Prof. W. Thomson (Kelvin) in which the 
theory of the discharge of a Leyden jar is discussed and the prediction 
made that under certain specified conditions the discharge must be oscil- 
latory. A number of similar papers going back to 1848 treat of similar 
subjects. Henry’s results do not appear to have become generally known, 
and we find the verification of Thomson’s prediction in 1857 by Feddersen. 
A number of other physicists have investigated the subject, the work of 
Schiller being of particular value. The recent applications will be referred 
to later. 
The mathematical theory of electrostatics and magnetism was greatly 
extended about this time by Thomson and others, and received its most 
complete statement at the hands of Maxwell in his papers read before 
the Royal Society and in his book published in 1873, still the standard of 
reference. Very little has since been discovered which was not fore- 
shadowed by Maxwell's theory or contained in his equations which have 
