1889.] Note on the Thermo-electric Position of Platinoid. 291 



silver, and slightly above the latter. It is, however, to be re- 

 membered that, in all probability, different specimens of platinoid 

 alloy would give results differing considerably from that quoted 

 above. 



Appendix. By A. Tanakadate. 



The following experiment on the torsional rigidity of spider line 

 was carried out in the Physical Laboratory of the Imperial University 

 of Japan, in 1884, and a notice of it was published in vol. 2 of 

 RigahuhyoTvwai Tassi ('Proceedings of the Science Society') of that year 

 in Japanese. It has not hitherto been described in English ; and the 

 absolute determination as referred to below by Mr. T. Gray of the 

 rigidity of silk fibre makes an estimate of the rigidity of spider line 

 possible. 



The determination of the torsional rigidity was a relative one, and 

 the experiment essentially consisted in finding the deflection of a 

 small magnet due to a given twist of the suspending fibre : the 

 magnet being placed in the earth's magnetic field (0 - 3 C.Gr.S.). The 

 deflection was observed by the usual method of the reflected image of 

 a fine wire stretched before a lamp. 



The mirror magnet was first hung by a silk fibre of 31 cm. length, 

 and placed in the usual way. The distance of scale from the mirror 

 was 95 cm. "When the torsion head of the magnetometer was turned 

 through one complete revolution (27r) in either direction from zero, 

 the image of the reflected wire was displaced through 8 mm. either 

 way, or 8/2 x 95=00042 radians, or 864". 



The silk fibre was now detached from the magnet, and a spider's 

 line (newly spun) was attached in its stead. The length was 28 cm., 

 the magnetometer was put into its place, and the torsion head was 

 turned as before, but no appreciable deflection could be observed, 

 even when the torsion head was turned through ten complete turns 

 (20tt). It was suspected then that the mirror might have been caught 

 against the sides of its case; a close inspection, however, showed 

 that it was quite free. The fibre was then shortened to 2'3 cm. 

 (about one-twelfth its previous length), and the experiment was re- 

 peated. Ten complete turns of the torsion head gave a deflection of 

 1*5 mm. ; or -15/2 x 95 = 0"00079 radians = 16-3" per turn. 



In order to compare these deflections with each other, each deflec- 

 tion was reduced to that which would be given by a fibre of 1 cm. in 

 length, by multiplying the deflections by the length of the fibre used. 

 Thus, corresponding to the twist of one turn of the torsion head in 

 a fibre of 1 cm. long, we have : — 



For silk fibre 864-0" x 31 = 26800" 



For spider line 16-3" x 2'3 = 37-5" 



