ON THE MODULUS OF TORSIONAL RIGIDITY OF METAL WIRES. 3 



in the course of this work yielded what seemed to he interesting information as to 

 the internal viscosity of the wires used. This is recorded and compared with similar 

 otwervations by other experimenters. The rest of the paper is divided into the 

 following sections: (I.) Description of apparatus, etc. ; (II.) Account of the experi- 

 ments; (III.) Summary <>f results and comparison with those of other observers; 

 (IV.) Determination of the coefficients of expansion of the wires. 



PART I. 

 DESCRIPTION OF APPARATUS, ETC. 



In these experiments the wire under test and the vibrator were both enclosed in a 

 heating jacket, the temperature of which could be regulated as required. This 

 arrangement was the result of many experiments to find the best method of overcoming 

 the various difficulties connected with obtaining a constant uniform temperature in 

 the wire. Some of these preliminary experiments will l)e referred to later, and the 

 reasons for rejecting old forms of the apparatus will be given. 



The part of the heating jacket surrounding the wire consists of two concentric 

 tubes (H, fig. 1). The outer one is a 3-inch brass tube, and is covered externally 

 with layers of cotton wool to a thickness of an inch all round. The inner tube is of 

 copper inch in thickness and 1 inch in internal diameter. This is joined to two 

 short lengths of wider copper tube at the top and bottom, in order to admit the 

 clamps for the wire. The inner tube of the jacket projects about l inches beyond 

 the outer one at the top, and is there held firmly in a vertical position by a strong 

 wrought-iron clamp, I, fixed to the wall of the laboratory, and projecting about 6 inches 

 from it. The heater is also supported by a larger clamp which grips the outer 

 cylindrical tube just above the box containing the vibrator. 



The vibrating plate hangs inside the double-walled copper box F. The floor of 

 this, requiring to be rigid, is made of -inch metal, but the other walls are not so 

 thick. The edges were, unfortunately, soldered together instead of being brazed, and 

 this caused many weeks of delay, the apparatus having to l>e repeatedly resoldered to 

 prevent leaking at the higher temperatures, due to unequal expansions tearing some 

 of the joints asunder. As will be seen from the figure, the double walls of this box 

 are in direct communication with those of the cylindrical part of the heater, so that 

 the steam by which the jacket is heated flows through the two. It is, of course, 

 necessary to have a door to the front of the box. This also has double walls and is 

 fed with steam by means of a side tube from the cylindrical part of the heater. The 

 waste-pipe from the door joins that from the bottom of the box, as is shown in the 

 figure. These pipes are made of J.-inch "composition" tubing and are joined on to 

 the door by means of brass " unions," a thin washer made of sheet asbestos being used 

 to make the joint tight. The door and the front- of the box on which it fits have 



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