1911-12.] Obituary Notices. 489 
A third form of experiment was devised by Chrystal himself, being a 
modification of one already tried by Schuster. It was based upon the fact 
that in an induction coil the induced current at break of the primary has 
a higher maximum intensity than the induced current at make. If, then, 
the induction currents from the secondary circuit of an induction coil, 
whose primary is made and broken by a tuning-fork, are passed through 
a galvanometer, the induced currents will not balance in their effects if 
resistance depends on strength of current. Certain effects, which at first (as 
in Schuster’s experiments) seemed to indicate a departure from Ohm’s law, 
were traced by Chrystal to the galvanometer. The explanation of these 
peculiar effects was given by Chrystal in a paper on “ Bi- and Uni-lateral 
Galvanometer Deflections,” which was published in the Philosophical 
Magazine for December 1876. Maxwell, writing to Tait on 5th February 
1876, put the results obtained by Chrystal in these words: “Ohm’s law 
has now been tested with currents that make the wire swag and swelter, 
and it is now at least 10 5 to 1 that if Schuster observed anything it was 
not an error of Ohm’s law.” As indicating the impression which Chrystal’s 
personality had made on Maxwell, the following quotation from a later 
letter to Tait is of interest. In the summer of 1878 Tait had evidently 
asked Maxwell for some help in conduction of heat calculations, and 
Maxwell replied : “ If you mean that I am, by the aid of Fourier, to get up 
the theory of a square box, and let you have it before the Edinburgh 
University Library opens, then in that case also you will not bother me, 
for I will not do it. Nevertheless, I have heard Chrystal say that the 
variable state of a parallelepiped was more tolerable than that of a cylinder, 
and he therefore cut his paraffin into a square prism. He also said that 
in this matter . Poisson was of more use than Fourier.” The most direct 
expression we have of Maxwell’s opinion of Chrystal’s capacity as an 
experimentalist is contained in the testimonial with which, on 10th July 
1877, he supported Chrystal’s application for the chair of Mathematics in 
St Andrews. “ Of Mr Chrystal’s papers,” he wrote, “ the most important 
is that on the c Testing and Verification of Ohm’s Law.’ . . . The difficulties 
which he encountered and overcame in the course of this work can be 
appreciated only by one who, like myself, has had opportunity of watching 
his progress through all its stages.” The testimonial ends with a reference 
to his “ extensive and thorough culture, his original and penetrating 
intellect, and his untiring energy.” 
No doubt it was on Maxwell’s recommendation that Chrystal was asked 
to contribute the electrical articles to the ninth edition of the Encyclopaedia 
Britannica ; and before he left Cambridge to take up his new duties in 
