(4) _The Difference Formula_ is the expression which gives the relation between gas-scale and platinum-scale temperatures, or _t_ - _p_ = δ{(_t_/100)^2 - (_t_/100)} This formula has already been fully dealt with. (5) _The Platinum Constant_ is δ in the above expression. The value for pure platinum is about 1·5, but small quantities of impurities may alter the figure considerably. The truth of the formula (4), however, is unaffected by changes in δ, as _p_ would be correspondingly altered. [Illustration: FIG. 33.—PLATINUM RESISTANCE PYROMETER.] =Practical Forms of Resistance Pyrometers.=—A typical form of resistance pyrometer, made by the Cambridge and Paul Instrument Company, is illustrated in fig. 33. The coil of platinum wire is wound round the edges of a mica framework, made of two strips of mica fastened at right angles so as to form a + in section. This method of winding is due to Callendar, who discovered that mica was chemically inert towards platinum, even at high temperatures. The leads, also of platinum wire, pass from the coil through mica washers to terminals fastened to the boxwood head. A second wire, not connected with the coil, but identical in length and diameter with the ordinary leads, is bent into two parallel branches, which are passed through the mica washers side by side with the leads, and are brought to a second pair of terminals in the head. The function of this wire is to compensate for changes in the resistance of the leads when heated, by opposing the compensating wire to the pyrometer in the measuring arrangement, when the resistance of the leads and wire, being equal, will cancel, the resistance actually measured being in consequence that of the coil only. Fig. 34 shows the connections for a Wheatstone bridge when this method of compensation is employed, _a_ and _b_ representing two equal fixed resistances, P the pyrometer coil, _x_ the leads, L the compensating wire, and _d_ the adjustable resistance. When no deflection is observed on the galvanometer, _a_ _x_ + P ──── = ──────── , _b_ L + _d_ and since _a_ = _b_ and _x_ = L, it follows that P = _d_. The protecting tube used by the Cambridge and Paul Instrument Company is made of porcelain, which is found to shield the platinum completely from the furnace gases, but is extremely fragile, and for workshop use should be protected by an outer iron sheath. [Illustration: FIG. 34.—WHEATSTONE BRIDGE AS USED WITH A RESISTANCE PYROMETER.]