48 
MR. F. E. SMITH ON THE ABSOLUTE MEASUREMENTS OF A 
is indebted to Dr. Rosa and other members of the Bureau of Standards. The 
instrument described below, which is of brass, was made by Mr. Murfitt of the 
Instrument Department of the Laboratory. 
Tire method of recording the speed is a direct one. A chronograph drum is geared 
to the main shaft of the apparatus through the medium of two worm wheels and two 
spur wheels, the gear ratio being 174. The usual speed of the main apparatus is 
about 1044 per minute, and under such conditions the drum of the chronograph makes 
one revolution in ten seconds. An electro-magnet is supported on a carriage which is 
connected to a split-nut engaging with a screw of 1 mm. pitch. The direction of 
travel of the carriage is parallel to the axis of the drum and it advances 1 mm. for 
each revolution of the drum. Every second a current passes through the electro¬ 
magnet and the latter operates a small punch, which, striking through a typewriting 
ribbon, prints a dot on a sheet of paper carried by the drum. The circumference of 
the drum is exactly 500 mm., so that when six revolutions per minute are made, 
successively recorded dots are 50 mm. apart. The split-nut fixed to the carriage 
carrying the electro-magnet can be disengaged from the driving screw and the carriage 
rapidly run along the rails supporting it. When the printing punch is pressed 
forward during this operation a line parallel to the direction of travel of the electro¬ 
magnet is printed on the paper. This line is hereafter called the base line and by 
measuring the angle between it and a row of dots recorded during a run, the speed 
can be calculated with great accuracy. 
The method of calculating the speed is as follows : At a speed of exactly 1044 
revolutions per minute there are 1044/174 = 6'0000 revolutions of the chronograph 
drum per minute. The resulting record will therefore be ten rows of dots, the dots 
being 1 mm. apart, and the rows 50 mm. apart and parallel to the base line. If the 
speed is slightly diminished or increased, the rows of dots will slope upwards or 
downwards. In practice, a set of observations for the measurement of a resistance is 
made to last for at least 1000 seconds (i.e., about 17 minutes), and the resulting rows 
of dots are therefore more than 10 cm. long. From a portion of the base line lying 
beneath (or above) a row of dots a length of 10 cm. is marked off, and from the 
extremities of the 10 cm. line ordinates are drawn to the nearest row of dots. If these 
ordinates are equal in length the rows are parallel to the base line, and the distance 
between two dots recording an interval of 1000 seconds (10 cm. run) is 50,000 mm. if 
the distance is measured along the trail of the recorded seconds. If the ordinates are 
not equal in length the machine was running either faster or slower than 1044 
revolutions per minute ; which of these holds good is decided by the direction of the 
slope. Suppose the difference between the ordinates is 14'5 mm. and that the machine 
was running faster than 1044 per minute. The mean speed is calculated to be 
1044 x 
50,000 
50,000-14A 
= 1044’30 3 revolutions per minute. 
It is clear that a difference 
between the ordinates of half a millimetre corresponds to 1 part in 100,000 of the 
