42 DRS. J. A. HAHKER AND P. CHAPPUIS ON A 



Here R and S represent the proportional coils of about five ohms each, adjusted to 

 exact equality. P is the thermometer coil connected by two long flexible copper ends 

 to the box terminals P, and P 2 . 



The wires in the stem of the thermometer leading to the coil are of thick platinum. 

 the coil itself being of a very pure sample of platinum "006 inch in diameter. Down 

 the stem run also a second pair of leads made as similar as possible to the coil leads. 

 but connected together at their lower extremities and having no contact with the coil. 

 This loop, connected at CjCj in the figure in the opposite arm of the bridge to the 

 thermometer itself, serves to compensate the changes in resistance of the thermometer 

 leads proper, due to variations of stem temperature. The four copper wires joining 

 P,Pj, CiC 2 to the thermometer are plaited together into a single cable, so that 

 temperature changes throughout their length may affect them all equally. 



Q represents the nine platinum-silver resistances of the box connected to one 

 another in series, the lowest coil having a resistance of 5 box-units (l box-unit = 

 '01 ohm), and the rest forming a series 10, 20, 40, 80, up to 640 units the largest 

 coil. An extra coil of 100 units is used to determine the fundamental interval of the 

 usual type of thermometer, whose change of resistance between and 100 is 100 

 box-units, i.e., 1 ohm. 



A platinum silver bridge-wire, AB, provided with a scale of millimetres, furnishes the 

 means of balancing exactly any resistance of P. A special form of slider is employed 

 for the contact between the bridge-wire and a precisely similar wire stretched parallel 

 to it, connected to the galvanometer. The exact position of the transverse wire 

 forming the contact-piece is indicated by a vernier by which '01 millim. may be 

 estimated. This symmetrical arrangement of two similar wires is found to diminish 

 the thermoelectric effects at the movable contact. 



Coils of 20 and 100 ohms are provided as resistances in the battery circuit, and also 

 a " tenth " shunt for the galvanometer. 



The top of the resistance-box is of a special quality of marble of good insulating 

 properties. 



The whole is enclosed in a double-walled tank holding a considerable mass of water, 

 which is kept at a constant temperature near 20, by a regulator controlling a small 

 gas flame. A delicate thermometer suspended in air in the interior of the box 

 indicates the coil-temperature, and the whole of the upper surface of the box is 

 protected against radiation and air currents by a glass cover similar to a balance case. 



IV. EXPERIMENTS WITH THE FlRST APPARATUS. 



From the time of the acquisition of this apparatus determinations were repeatedly 

 made of the constants of each of the platinum thermometers, in order to test the 

 permanence of the whole arrangement under ordinary working conditions ; also to 

 ascertain how the accuracy obtained was influenced by alterations in the external 



