66 Mr. Hollo Appleyard on a 



" dial " bridge was therefore abandoned, and a slide-bridge 

 of special form was designed for the purpose. 



Measurement of H f by Slide-bridge. 



The slide-bridge, fig. 1, consists of a 12-ft. plank, P, with 

 a groove G cut along its whole length. A metre-scale, S, 

 can slide in the groove, flush with the upper surface of the 

 plank. Datum-marks are made at successive metres along 

 the plank ; subdivisions being read upon the sliding metre- 

 scale. The contact-piece, L, is attached to the metre-scale at 

 its zero-point. This saves the cost of an elaborate graduation 

 of the plank ; and keeps the sliding-contact very accurately 

 in position. When necessary, the metre-scale with its con- 

 tact-piece is easily transferred to other such slide-bridges. 

 There is no distortion of the slide- wire, and indentation is 

 almost impossible*. The slide-wire, W, is of platinoid f 

 stretched between two adjustable blocks of copper, B. Its 

 ends are soldered into saw-cuts made in the copper blocks. 

 On the other side of the scale-groove there is a copper wire, 

 J, for the galvanometer-contact. The contact-piece consists 

 of an ebonite block K, with a gap at one end. The platinoid 

 wire slides through a saw-cut M transversely to this gap in 

 the ebonite. A Y-shaped piece of spring-brass, L, is screwed 

 to the ebonite block. One end of the Y, projecting over the 

 gap, is provided at !N" with a platinum knife-edge. The other 

 two arms of the Y project beyond the other end of the ebonite 

 block, and carry clips, R, through which the wire for the 

 galvanometer-contact slides. 



The bridge-coils, fig. 2, consist of five silk- covered platinoid 

 resistance-bobbins, immersed in a double vessel of copper, 

 the inner vessel containing 1 J gallons of paraffin-oil. 



A glow-lamp, submerged in the oil, is used for maintaining 

 the temperature constant at 80° F.; this temperature was 

 chosen as being just above ordinary air-temperature. 



Copper rods (a, b, c, e,f,g, k) connect the coils ; their 

 ends pass upward, through the wooden cover of the calori- 

 meter, to the switch. 



Fig. 2 shows the arrangement of these coils. The two 

 ends of the slide-wire go to / and g respectively ; to which 

 points the 100-ohm coils (/, e) and (g, a) are connected. 

 By switching p to I, the fixed arm (45 ohms) of the bridge, 



* A very convenient optical bench could be made by such a grooved 

 plank. 



t Platinoid is not good for slide-bridge wires. After about two 

 months' exposure this wire has become blackened, and I am replacing it 

 by a wire of platinum-silver. 



