88 BRIDGMAN. 



itself by three steel pillars 0.062 inch in diameter at either end. These 

 pillars passed through washers of transparent bakelite at either end. 

 At the lower end the bakelite washer was in turn attached to a thin 

 steel sleeve, which was attached by screws to the three terminal plug. 

 The screws connecting the bakelite washer to the steel sleeve were 

 made a loose fit in order to allow sufficient freedom for the german 

 silver springs to keep the specimen concentric in the cylinder. The 

 german silver springs were soldered to thin discs of german silver 

 drilled to slip over the three steel pillars at either end and held in 

 position by the same nuts which held the bakelite washers. The steel 

 sleeve was cut away on two sides, allowing the ends of the thermo- 

 couple and the heating unit to be brought through the bakelite washer 

 and soldered to the three terminal plug. As an additional precaution 

 against leakage of heat from the heating element into the thermo- 

 couple, two massive pieces of copper in the form of a double wall 

 were placed entirely across the pressure cylinder between the terminals 

 of the plug connecting the thermo-couple and those connecting to the 

 heating element. 



Two forms of heating element were used for these cylindrical 

 specimens, according to whether there was an axial tube, or the 

 heating element was mounted in a fine hole cast along the axis of the 

 specimen. When the tube was used, the heating element was in the 

 form of a hairpin of 0.005 inch nichrome wire. By means of a jig the 

 wire was made accurately 5 inches long when straightened out, so 

 that it would exactly occupy the length of the tube when bent double. 

 It was silver soldered at either end to a half-round piece of copper 

 filed from a | inch copper wire. The whole arrangement was covered 

 with five or six coats of insulating enamel baked on, having a thickness 

 of about 0.0008 inch. After enamelling, the wire was bent double 

 into the hairpin form, thus bringing together the two halves of the 

 copper wire so as to form a single completely round piece | inch in 

 diameter. As an additional precaution the two halves of copper were 

 frequently insulated from each other by a thin layer of mica. The 

 two halves were then bound together into a single piece by wrapping 

 with fine silk thread. The resistance of the copper terminal pieces is 

 so much less than that of the nichrome element itself that there is only 

 a very small error introduced by any generation of heat outside the 

 element. The massive copper pieces were attached to the terminals 

 of the three terminal plug by a short length of flexible conductor 

 twisted out of many fine strands of copper. The total resistance of 

 the heating elements made in this way was nearly 10 ohms. 



