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The Radiometer

THIS instrument, invented a few years ago by Professor Crookes, is certainly one of the most astonishing in its results of any that has appeared even in this century of scientific wonders. It consists of four vanes of mica, each blackened on one side. These vanes are fixed together on a tender wire framework, so arranged with a point in the centre, that they hang free to rotate round that point which rests in a glass cup at the top of a central stem of the same material. The whole is inclosed in a glass bulb which has been exhausted of air to the greatest possible extent. A reference to the cut will make these several parts readily understood.

Directly this little instrument is placed before any source of light, the little vanes begin to rotate, and will go on moving until the light is withdrawn. The inventor of this marvellous little instrument gives the following explanation of its movements:- "The light, or the total bundle of rays included in the term 'light,' falling upon the blackened side of the vanes, becomes absorbed, and therefore raises the temperature of the black side; this causes extra excitement of the air molecules which come in contact with it, and pressure is produced, causing the fly of the radiometer to turn round."