THE AID OF THE ACHROMATIC FRINGES. 



45 



proach of g to *, the vibration becomes unstable or shows a tendency to become 

 torsional. The rather thick wires used are an advantage in this respect, as 

 they prevent any sticking of the lens-carriage to the magnet and serious tremor 

 during manipulation. 



The method of observation consists in gradually increasing the tension of 

 the wire from a slightly low value to beyond the maximum tension. In this 

 case the band widens from a relatively small width to the maximum and then 

 (as a rule) abruptly falls off to a small value. To repeat the observation the 

 tension must often again be reduced to the low value and the whole operation 

 repeated; but after some practice, maxima may be reached in the reverse 

 direction unless the image band is too far spent and narrow. 



When the current is broken and thereafter closed, a low width of band is 

 obtained which will not usually widen unless the operation described is carried 

 out from low tension. In other words, there are two cases of equilibrium for 

 each current, corresponding to very different image-band widths. This is a 

 curious result, for it means, virtually, that the magnetic forces and the stresses 

 are in the relation of a doubly inflected curve to each other, so that there are 

 three intersections, two for stable vibratory equilibrium; or else the two har- 

 monic systems, the electrical and the mechanical, may vibrate in the same or 

 in opposed phases, with the latter preferred. Figure 45 gives a diagrammatic 

 comparison of the tensions and the band- widths, s; the maxima are at m and 

 m' . The arrows indicate the cyclic result of change of tension. 



Similarly each current requires its own particular maximum tension, which 

 increases with the current. The difference is not large, but very operative, 

 and for this reason the fine tension adjustment is essential. 



2< 



29. Further observations. The apparatus was then improved in a variety 

 of ways, chiefly by the insertion of a small vibration objective about i cm. 

 in diameter, achromatic and with a focal distance of but 5.8 cm. In this 

 case the distance a and b could be decreased to 7 cm. and 35 cm. and the ob- 

 server was thus conveniently near the adjusting-screw. The slit-image was 

 white and about a scale-part in width. There would have been no difficulty 

 in using much greater magnification. 



TABLE 4. Comparison of Siemens (<p) and telephonic dynamometer. C = io~ 3 Xo.87; ^ = 710 

 ohms; i = C^,<p; = 7 cm.; b =35 cm.; / = 5-8 cm. Phosphor-bronze wires 50 cm. long; 

 0.025 cm - m diameter. 



