TRANSACTIONS OF SECTION A, 



547 



kind. The dynamical theory of the steady motion observed in all such cases was 

 «xplainedto the Section. 



9. Artificial Production and Maintenance of a Standing Bore. 

 By Sir William Thomson, LL.D., F.B.S. 



10. Velocity of Advance of a Natural Bore. 

 By Sir William Thomson, LL.D., F.B.S. 



11. Graphical Illustrations of Dee}) Sea Wave-groups. 

 By Sir William Thomson, LL.D., F.B.S. 



12. Sir William TJiomson's Improved Wheatstone's BJieostat. 

 By J. T. BoTTOMLET, M.A., F.B.S. E. 



Wheatstone's rheostat was invented over forty years ago ; but, though admirable 

 in conception, and commonly shown on the lecture-table in explaining the nature 

 of and illustrating electric resistance, it is scarcely if at all used in the laboratory. 

 This is altogether owing to practical defects in the instruments as commonly con- 

 structed. The wire comes loose, the contacts are uncertain, and, the current beino- 

 incessantly broken and made again, the galvanometer needle is perpetually swino-ino- 

 about, instead of showing, as it ought, a continuously increasing or diminishing 

 deflection when the resistance is wound out of the circuit or wound into it. 



Modifications of the original instrument have been made from time to time 

 and a very important improvement was recently introduced by Mr. Jolin, of 

 Bristol. In John's rheostat a toothed-wheel fixed on one of the two cylinders gears 

 into a toothed wheel on a shaft carrying the other cylinder, and a spring fixed to 

 this shaft acts on the last-named cylinder, which surrounds it, on the principle 

 of the mainspring of a watch. By this arrangement the wire is kept tightly 

 stretched, and the barrels can be turned both forwards and backwards by means of 

 .a handle attached to one of them. Thus the necessity of shifting the handle from 

 one to the other when the motion is to be reversed is obviated. 



In Sir William Thomson's improved rheostat the spiral groove in the non- 

 conducting cylinder of previous instruments is dispensed with, and the wire is 

 guided between the cylinders so as to be laid on them spirally by means of a travel- 

 ling nut on a long screw. The screw is turned by the handle and carries a toothed- 

 wheel which gears intotwo toothed-wheels, one of which turns one of the cylinders, 

 iind the other the axial shaft of the other cylinder containing the watch-sprino-. 

 The guiding nut is also arranged to stop the motion of the screw-shaft at each en°d 



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