EECENT PROGRESS IX PHYSICS. 



347 



Fis. 63. 



Riess, in order to shorten the platinum wire -which closed the main 

 Spiral, used Berguet's metallic thermometer instead of the air ther- 

 mometer, 



• A straight platinum wire 61.5 lines long and 0.04 line radius was 

 fastened immovably in the axis of a sensitive thermometric spiral, 

 similar to that represented in fig. 63, and intro- 

 duced into the circuit in a suitable manner. The 

 [instrument was of course placed under a bell- 

 glass. The platinum wire in the axis, on being 

 'heated by a discharge of the battery, commu- 

 •nicated its heat to the spiral ; the index then 

 liraversed a number of degrees, but soon returned 

 :to its first position, in consequence of the rapid 

 (cooling caused by the large volume of air in the 

 Jbell-glass. 



I The experiments with the metallic thermometer 

 iteach nothing new, on which account no further mention need be made 

 lof them, though I could not leave this method of observing unnoticed. 



I § 66. Direction of the secondary current.— -T^o investigate whether 

 ;the direction of the lateral current changes with the distance of the 

 ^secondary wire from the main wire, Riess used the following method. 

 '(Pog. Ann., LXXI, 351.) 



Au insulator, which cannot be pierced by electricity, being placed 



between the free ends of the secondary spiral, no secondary current 



occurs. Nevertheless the electrical equilibrium of the secondary wire 

 [is destroyed by the act which would have produced the current, as the 



following experiment shows : 



I If we place between the free ends of the p.i„ ^4 



[secondary spiral a thin cake ot resin, so 



^that the two ends of the wire are opposed 



[to each other, after the discharge of the 



[battery by the main line, the two surfaces 



[of the cake of resin may be distinguished 



;from each other in the most decided man- 



'ner. Peculiar electrical figures are pro- 



'duced, which, in most cases, are brought 



out by slightly breathing upon them. If 

 jit be desired to fix the figures, it is done, 



as shown by Lichtenberg, by strewing the 



surfaces with a mixture of flowers of sul- 

 iphur and minium. On one of the sur- 

 ■ faces of the resin treated in this way there 

 'appears a red disk, with a red border, and beyond it a dark (unpow- 

 ;dered) ring, surrounded by yellow rays. On the other surface yellow 

 'and red segments of circles are visible, embraced by a wide red ring. 

 The rays and the ring increase with the 



strength of the electrical excitation; with 

 ' very feeble excitation the rays of the first 



figure are wanting, and a simple red disk 

 r remains, which, however, is sufficiently dis- 

 \ tinct from the second figure, in which the 

 I red ring may always be recognized. 



Fig. 65. 



FiR. 66. 





