With a very small knob of wax fasten a pin to each face, as shown in the figure. Having done this place a lighted candle within the glass.
The heat, striking the interior equally, is modified by the colors painted on the outside to such an extent that after some little while the wax supporting the pins of the BLACK faces is melted, whilst the pins on the white parts remain unaffected. This shows very clearly that the white prevents the escape of internal heat, as surely as it prevents the penetration of external warmth.
The Pyrometer
We all know that metals expand under heat. The amount of such expansion may be measured by a simple little apparatus called a pyrometer.
Fig. 13.—Showing how to make a pyrometer.
On a wooden base, B, C ([Fig. 13]), make two uprights, A and D, of which A must be a half inch higher than D. Bore a hole a quarter of an inch from the top of A, but not right through the wood.
A couple of pins must be bent into the shape of a Y and driven into the top of D, as in [Fig. 13]. With a little sealing-wax fasten a paper pointer to the eye end of a needle and lay the needle across the pins, P, P ([Fig. 14]). Next place an ordinary knitting-needle in the hole at A, and rest it over the small needle with the pointer. The pyrometer is now complete.
Put a lighted candle under the knitting-needle between D and A, as in the figure, taking care that the flame plays freely upon the needle. As the latter grows hot you will notice that the pointer moves slowly from left to right, being acted upon by the hot knitting-needle passing over the axle at X.
