Glass, and Metal Wires subjected to Constant Pull. 523 



roll of wire, and no consistent results were obtained until 

 wires were used which came from the same roll, and which 

 were annealed at the same temperature for the same length of 

 time, and were cooled at the same rate. 



Having no convenient annealing furnace, I annealed the 

 wires separately by an electric current. The whole annealing 

 apparatus is represented diagrammatically in fig. 2. 



PK 



MR 



l-f^ 



A represents the accumulator- cells which supply the current. 



"W is a Weston standard ammeter and voltmeter. 



S is the wire which is to be stretched. It is enclosed in a 

 glass tube to prevent air draughts, and when copper wires 

 were used the glass tube was filled with coal-gas to pre- 

 vent the oxidation of the wire. 



CR is a carbon rheostat for adjusting the current. 



MR is a rheostat for cutting off the current at the same rate 

 at each annealing. It consists of two concentric tubes, 

 the outer of glass and the inner of porcelain. The lower 

 end is closed with a bung, and at the botton of the glass 

 tube is a side tube with a tap. Round the inner tube is 

 wound a long spiral of iron wire of large resistance com- 

 pared with the rest of the circuit, and connected up as 

 shown. The space between the two tubes is filled with 

 mercury while the annealing continues. At the required 

 time the tap is turned on and the mercury slowly flows 

 out. thus putting in gradually more and more resistance. 



PK is a plug-key. 



It was found that the way in which the load was put on 



made some difference to the result. 



Ultimately uniformity was obtained by using a flotation 



method which put the full load on in about 5 seconds. 



2M2 



