492 
Proceedings of the Royal Society of Edinburgh. [Sess. 
In these days in the Scottish Universities, there were during summer 
no regularly constituted classes in the Arts Faculty. Consequently, freed 
from the trammels of class work by the beginning of April, Chrystal found 
opportunity to resume with eagerness his experimental investigations, from 
which his St Andrews career had completely divorced him. Tait invited 
him to work in the Physical Laboratory and to utilise to the full all its 
appliances. It was my first year as Tait’s assistant, and the incursion of 
this young professor of twenty-eight years into our midst gave all our 
minds a new orientation. His constant presence in the laboratory during 
the summer months and his ready accessibility at all times gave a great 
impetus to the experimental study of electricity and magnetism. Tait 
himself was at the time fully occupied with the corrections to be applied to 
the Challenger thermometers and with the related work on high pressures. 
This work was being done in the basement by a few of the senior students 
working directly under Tait’s supervision ; and Tait was rarely seen in 
the upper rooms where most of the other laboratory work was going on. 
Summer after summer Chrystal flitted through these laboratories, busy with 
his own researches, but not too busy to take a keen interest in all that was 
being done. Many a helpful suggestion he gave for new lines of work, and 
many an eager student did he encourage by inviting his co-operation in 
some special bit of investigation. The advanced students of these years 
came into more direct contact with him than with Tait, and owed much of 
their scientific progress to his sympathetic help. My own research work in 
magnetism, which has continued over many years, had its origin in a con- 
versation over a passage in the article “ Magnetism.” 
The first work to which Chrystal devoted himself was the comparison 
of inductances and capacities according to the methods soon to be expounded 
in his important paper on the differential telephone, for which he was 
awarded the Keith prize. This paper, indeed, contains for the first time 
the complete theory of the Wheatstone bridge through which a periodic 
current is passing, when coefficients of mutual and self-induction are given 
their full significance. In the course of his investigations he constructed 
many forms of apparatus, ^hich were the pioneers of the more elaborate 
and refined methods of the present day. Side issues of enticing interest 
often led him away for a time from the main trend of his researches. On 
one of these, the wire telephone, he made two communications to our 
Society, and showed some of the experiments. The accounts of these in 
our Proceedings are mere abstracts ; a much fuller description will be 
found in Nature of 29th July 1880. 
During this first strenuous year at Edinburgh, Chrystal also presented 
