360 MESSRS. R. T. GLAZEBROOK AND T. C. F1TZPATRICK 



to have resistances of very approximately 1 B.A. unit, 1 legal ohm, ^ B.A. unit, and 

 B.A. unit respectively. The ends of the tubes were ground square by placing each 

 in a groove in a piece of hard wood and allowing the end to pass through a hole 

 which it accurately fitted in a brass plate fixed at right angles to the groove. 



The tube was made to rotate slowly round its axis, and the end ground by emery 

 on a copper plate. 



To find the value of the length (L), two small rectangular pieces of brass were 

 used. These were carefully squared, and had a fine X engraved on a small piece of 

 white metal inserted in the centre of one of the faces of each block. 



The tube was placed under two reading microscopes, which could be adjusted 

 longitudinally by micrometer sci'ews, graduated to '00002 inch, and one of the 

 rectangular pieces of brass was brought up to each end, and adjusted so that one 

 edge touched the tube as nearly as could be along the horizontal diameter while the 

 cross mark on the brass lay on the axis of the tube produced. The microscopes were 

 then focuseed on the crosses, and several readings taken as the tube was turned 

 round on its axis; except in the case of one tube (No. III.), no appreciable 

 difference was observed on turning the tubes round, and in the case of III. the 

 difference did not amount to '004 centimetre. 



The tube and brass pieces were then removed and their places taken by the 

 standard metre of the Cavendish Laboratory. 



This is a bronze rod 1 metre long, divided into decimetres ; the first decimetre is 

 divided into millimetres. The length of the rod, as determined at the Standards Office 

 of the Board of Trade by comparison with the standard metre, S.S., is 99'995 cm. 

 i '001 at C., while its coefficient of expansion is '000017 per 1 C. 



The first decimetre and the millimetre subdivisions have no appreciable error. 



For the lengths greater than one metre a second bar, made by the Cambridge 

 Scientific Instrument Company, was used. This was 30 cm. in length, and was 

 divided into decimetres, which were compared with those of the standard. 



The two bars were placed end to end in a long wooden box, which they fitted 

 tightly, and the distance between the last division of the standard and the first 

 division of the bar measured with the microscopes. 



The temperatures were read by a thermometer laid on the scale or against the 

 glass tube. 



The length thus found was that of the glass tube, together with the sum of the 

 distances between the cross marks on the brass blocks and their edges which were in 

 contact with the tube. This distance was carefully determined by aid of the reading 

 microscopes and the standard metre. 



The values found for this correction differed at most by '005 cm., and the obser- 

 vations on the length of the tubes, which were repeated in each case three or four 

 times on separate occasions, agreed to about the same amount. Thus, the error in the 

 length of the tubes is probably in no case greater than '002 cm. in a length of about 

 100 cm. 



