so.ur. ro.v77:A//'('A'./A'r .ii>r.i.\ci s i\ riivsics /;• 



•17.1 



ver\' lonj;, thi-n llu- ciirvi- ohtaiiu-d liy displarini; llio iiidwiIiIl- mirror 

 step by step and plotting the receiver-reading against tlie inir'-or- 

 displacenieni would l)e a perfect sine-curve; the ilistaiice between the 

 positions of the mirror corresponding to two consecutive maxima of 

 the ciir\e woidd be half the wavelength of the wave-train. Unfortu- 

 nateK' neither the recei\er nor the wa\e-train is ever perfect. The 

 wave-train is a heavily-damptxl sinusoid, and consequently the curve 

 of receiver-reailing \ersus mirror-displacement flattens out before the 

 mirror has l)cen moved very far. Kven so, the interpretation might 

 not Ix? imcertain if the receiver gave a reading proportional to the 

 time integral of the intensity of the wa\-e-moti()n at M. This it 

 rarely does. 



The receiver, in this region of the spectrum, must be a thermal 

 receiver — a short thin wire or a narrow band of sputtered metal upon 

 a strip of insulating substance, or sometimes a wire loop. In this the 



\'i^. 5 — Thirmoflcctric Receiver Used by .Mobius. (Anmilen der Physik) 



incident wa\es induce a resonance-current, of which the Joule heat 

 produces the directly-measured effect. -A thermojunction may be 

 intercalated in the resonant wire, as in Mobius' apparatus (Kig. 5; 

 in the middle of the transverse piece, 14 mm. long and 0.3 mm. thick, 

 a platinum tip is welded into a tellurium socket). Nichols and Tear, 

 developing a method introduced by G. F. Hull, mounted the thin 

 wire or the sputtered ribbon in front of a radiometer- vane; the Joule 

 heat warmed the front face of the vane, and the rather mysterious 

 agencies sometimes called '"radiometer forces" came into play. Four 

 of their receivers are shown in Fig. 6 at b, c, d, and e. In each of 

 these sketches I'l represents the edge of the radiometer vane; e in 

 sketch fc is a wire running from end to end of it, while e^, €■>, etc., in 

 sketches d and e are short wires mounted vertically or horizontally 

 l)ehind it. The mounting is shown in Fig. 6a; the vanes are seen 

 front-face, one having its wire or wires in front and the other l)ehind, 

 so that the radiometer forces on both will produce torques acting 

 in the same sense. The vanes with the cross-pieces Cy and d are 

 mounted upon the rod q, which is suspended from a torsion-fibre; 



