156 Intelligence and Miscellaneous Articles. 



ming grows more and more feeble. At last, if air be withdrawn 

 until only a pressure of 3 or 4 millims. is left in the tube, the ear 

 can no longer perceive any thing ; but a brilliant white light ap- 

 pears, springing in the form of beads from the tinfoil of the con- 

 denser, and absolutely distinct from the pale and indistinct phos- 

 phorescent light of Geissler tubes. — Cornjptes Renclus cle VAcademie 

 des Sciences, Jan. 5, 1880, p. 36. 



ON THOMSON S GALVANOMETER. BY A. GAIFFE. 



When electric currents are measured with the aid of Thomson's 

 reflecting galvanometer, the indications read on its graduated scale 

 are found not to be proportional to the values of the currents mea- 

 sured, and to be exaggerated in proportion as those values increase. 



This cause of error arises from the fact that the angles of deflec- 

 tion of the magnetized needle are doubled by the reflection of the 

 mirror, and consequently it is not the tangents of the real angles of 

 deflection that one reads on the scale, but the tangents of the 

 doubled angles. If it can be assumed without inconvenience that 

 the tangent of the double of a very small angle is twice that of the 

 simple angle, the case is not the same for large angles ; and although 

 the deflections of the needles of this class of apparatus are limited, 

 with the aid either of a directing magnet or of derivations, to about 

 8°, the indications are already sensibly exaggerated at the extremi- 

 ties of the scale which the 8° include. 



I essayed to remedy this defect by substituting a very fine plati- 

 num wire (yJo o" i n diameter) for the untwisted thread of silk by 

 which the system of needles of the galvanometer are suspended; 

 but the want of proportionality was produced in the opposite sense, 

 and was more considerable. 



I then had recourse to a bifilar suspension composed of two 

 cocoon-threads very close to one another. The results were almost 

 completely satisfactory. By this second means the errors are not 

 entirely corrected ; but they are reduced to less than one hundredth 

 of the value measured. 



The verification-experiments were made, using the whole extent 

 of the graduated scale, with the aid of three condensers charged 

 from one and the same constant source of electricity. The con- 

 densers were first charged and measured separately, and afterwards 

 charged and measured in combination. — Oonvptes Renclus cle VAca- 

 demie des Sciences, Jan. 12, 1880, t. xc. p. 94. 



TIN-MINING IN BANKA AND BILITONG. 

 In the ester reichische Zeitschrift fur Berg- und Huttenwesen, 

 vol. xxvii. Jahrgang 1879, Dr. Ed. Beyer has given a concise, though 

 well detailed, notice of the nature and condition of the tin-mining 

 in Banka and Bilitong, with a geological sketch map of the north- 

 east district of the former island, and full references to former 

 observers and writers on these works and the structure of the 

 islands. 



