Page:Collected Physical Papers.djvu/379

Rh spot of light now rushed across the whole length of the scale in $1 1⁄2$ seconds, growth being enhanced to more than three times the normal rate. The entire series of experiments on the effect of variation of temperature on growth was completed in the course of 15 minutes.

Experimental demonstrations of the action of light, and of various chemical agents can also be given in a similar manner.

As an example of the application of the method of high magnification in physical investigations, I describe the Magnetic Radiometer in determination of the energy of different rays of the solar spectrum by measuring the elongation of a metallic wire coated with lamp-black. The particular spectral ray falling on the wire is absorbed, and thus raises the temperature proportionately to the energy of radiation. The resulting increase of length is so minute as to be undetectable by any method of magnification hitherto available.

A diagrammatic representation of the apparatus is given in fig. 114. W is a length of zinc wire which becomes lengthened by rise of temperature produced by absorbed radiation. It is attached by a hook to the short arm of a long magnetic lever, the N end of which is lowered by any elongation of the sensitive wire. In front of the N end of the magnetic lever is suspended a small magnetic needle with an attached mirror. As the N pole of the magnetic lever is lowered, it produces an increasing deflection of the suspended needle which is magnified by the spot of