38f> 



Special Reports 



Table I. Diameters. 



The mean overall diameter of each spiral was obtained to 0.001 mm. from the curves 

 of Figure 15 by means of an Amsler planimeter. By subtracting from this the diameter 

 of the wire the mean diameter of the spiral was obtained. 



The mean diameters are given in Table I. 



The 30 cm. standard to which the diam- 

 etral measurements were referred was correct 

 to 1m- From this fact and the way in which 

 the other measurements were made, the total 

 error in the mean diameters given in the 

 table can hardly equal 4^. 



16. The axial distances between the cor- 

 responding spirals of the Helmholtz coils 

 were measured with a Geneva standard 

 comparator and a special millimeter scale 

 ruled in this laboratory and calibrated at the Bureau of Standards. 



For this purpose the comparator had to be somewhat modified. The bases for 

 holding the scales were removed and a slide with longitudinal screw motion was screwed 

 to the movable bed with the direction of motion of the slide normal to that of the bed. 

 On this slide was fastened a simple frame in which the spool was held with its axis hor- 

 izontal and with rotation about this axis possible. The microscopes were raised on brass 

 pillars until they were high enough to focus properly upon the top of the coil when in 

 its frame, and were clamped in such positions that when the slide was in the center of its 

 range the mean planes of the two Helmholtz coils were nearly in the optic axes. The scale 



'19 900 





I4&90C 



149.900 



A 2-3 



A 1-4 



B2-3 



B 1-4 



C 2-3 



C 1-4 



D2-3 



D 1-4 



23456789 

 SPIRAL NUMBER 



I4990O 



^ 



149.900 



149.900 



10 I 



A 2-3 



A 1-4 



B 2-3 



B 1-4 



C 2-3 



C 1-4 



D 2-3 



D' 1-4 



2 3 4-56 78 



SPIRAL NUMBER 



10 



Fig. 16. Axial Distances between Corresponding Turns of the Spirals. 



was applied to the coil with its edge nearly in the vertical plane through its axis. The 

 scale had a wedge-shaped edge which very nearly touched the wires, the plane of the 

 divisions cutting the wires at a short distance. Such adjustments were made that when 

 the comparator carriage was moved from one stop to the other the microscopes were 

 focussed either on the edge of the scale or a central section of the wire. By means of the 

 slide two corresponding wires (150 mm. apart) were brought into the microscope fields 

 and the slide and microscopes adjusted until the images were nearly central. Micro- 



