In London his duties were not so congenial as they had been in Aberdeen. The session was much longer, and he was not so free to adopt his own methods, for the college was affiliated to the University of London, which alone had the power of granting degrees. After five years in this office he retired to his own estate. While in London he carried out three important investigations. He had already investigated the mixing of colors reflected from colored papers; he now took up the mixing of pure colors of the spectrum. For this purpose he made a wooden box 8 feet long, painted it black both inside and outside, fitted it with the necessary slits, prisms, and lenses; and, in order to get the necessary sunlight, placed it in the window of the garret of his house. Here he observed the effect of mixing the spectral tints, and his neighbors thought him mad to spend so many hours staring into a coffin.
His investigation of the stability of Saturn's rings introduced to his attention the flight of a countless horde of small solid bodies; from this to the kinetic theory of gases the transition is natural.
The third task was the construction for the British Association of a material ohm, defined as the resistance of a circuit when an electromotive force of one volt sends a current of one weber through it. Maxwell, more than any other man, was the founder of the C.G.S. system of units, which became the basis of that practical system of electrical units which is now legalized in all civilized countries. "Weber" was originally the name for a unit of current. In the last verse of his "Valentine from a male telegraphist to a female telegraphist," Maxwell introduces the newly defined units:
And clicked the answer back to me,—
I am thy farad, staunch and true
Charged to a volt with love for thee.
It was eminently appropriate that in 1900 the International Electrical Congress should give Maxwell's own name to the unit of magnetic flux.
