276 Profs. Dewar and Fleming on the Electrical 



a 



§ 3. Passing on to further details, Ave proceed to describe 

 briefly the preparation of the metals and alloys in the required 

 state of purity, and the construction of the resistance-coils 

 suitable for the purpose of these experiments and the mode of 

 making the required measurements. Our previous experi- 

 ments had shown the necessity for the greatest care in the 

 preparation of the metals. The term pure is of course a 

 relative term, and absolute purity in a chemical sense is of 

 course a most difficult matter to secure, but every care has 

 been taken to arrive at the greatest possible purity in the 

 majority of the metals employed. In this part of the work 

 we have been greatly aided by several friends. Foremost 

 amongst these we are especially indebted to Mr. George 

 Matthey, who has given us invaluable aid in the preparation 

 of a large series of metals in a state of great purity. Mr. J. 

 W. Swan has furnished us with many samples of electrolytic 

 copper and silver prepared by himself by his own special 

 processes. From Mr. Gr. Addenbrooke we received samples 

 of aluminium and aluminium alloys which have been of use 

 to us ; and Colonel Dyer, of the Els wick Ordnance (Arm- 

 strong's) Works, has kindly sent us samples of iron prepared 

 with very great care. 



Having these materials in our possession, they were in 

 most cases drawn into wire for us by Messrs. Johnson and 

 Matthey, and in some cases, such as those of lead, tin, thal- 

 lium, and some others, the metals were pressed into wire in a 

 steel press by the hydraulic 50-ton testing-machine in the 

 Mechanical Laboratory of University College, London. 



These different wires had a diameter varying from *08 

 millim. (3 mils) up to about 025 millim. (10 mils) and 

 lengths of from 1 to 3 metres. 



§ 4. The next step consisted in measuring the mean dia- 

 meters. For such fine wires no ordinary process of callipering 

 was sufficiently accurate. After many experiments, the 

 following method of obtaining rapidly the mean diameter was 

 perfected. 



A compound microscope had the eyepiece removed, and the 

 substage under the object-glass fitted with a large cork, bored 

 out in the centre with a hole, and having two small glass 

 tubes inserted in the sides. This cork was so fixed to the 

 stage that when a wire was drawn through the small tubes, it 

 passed across the field of the objective. The stage had a 

 rack-work motion which enabled it to be moved to or from 

 the objective ; but this last remained fixed relatively to the 

 stand of the microscope. The microscope being placed hori- 

 zontally, an incandescent lamp was placed behind a condenser 



