54 
MR. F. E. SMITH ON THE ABSOLUTE MEASUREMENTS OF A 
by means of coiled springs contained in the barrel. It was found, however, that this 
arrangement gave rise to a greater pressure than was desirable at the point of contact 
between the plunger and the coil. Owing to the small radius of the wire with which 
the coils are wound, a comparatively slight pressure would produce deformation 
sufficient to lead to errors in the readings. A calculation showed that in order to 
keep errors due to this cause within allowable limits (less than rooo nim. on the 
diameter) the pressure must not exceed four or five ounces. At the suggestion of 
Mr. J. E. Sears, the springs were taken out and the vertical lever was extended 
downwards and provided with a weight at its lower end, thus forming it into a 
pendulum with a gravity control which could be adjusted to a nicety. At the same 
time the form of the body of the plunger was slightly modified, and its hearing 
surfaces lightly polished, so as to minimize friction. In this way the contact pressure 
was reduced to within the required limits. 
Many measurements of the coils were made with a current passing through them 
in order to reproduce as nearly as possible the conditions under which they would 
actually be used. To enable this to be done, the plungers were provided with non¬ 
conducting measuring faces of polished agate. These proved highly satisfactory in 
use, and the measurements made with the machine were of quite remarkable 
accuracy. 
Two pairs of Hartmann steel gauges were employed in the measurements. The 
following table gives their values at 0° C. :— 
Table II.—-Giving the Values of the Gauges employed.* 
Approximate length of 
gauge in millimetres. 
Sevres value. 
N.P.L. determination, 
using the Sevres value of 
the 1000 mm. gauge 
as basis. 
60 
mm. /a 
60-1*5 
mm. [x 
60-1-3 
160 
160-0-7 
160-0-7 
200 
200 + 0-1 
200 + 0-4 
300 
300- 1-1 
300-0-9 
By addition we get 
Gauges 300+ 60 
„ 200 + 160 
Sevres value, 
mm. /a 
360-2T 
360-0-6 
N.P.L. value, 
mm. n 
360-2-2 
360-0-3 
The Sevres values are stated to be accurate within 0and the probable error of 
* 
/X = 
1 micron = 0 - 001 mm. 
