788 



KEPORT — 1898. 



sides and bottom. Water in the jacket was lieated by a Bunsen burner put belo-w 

 the can. A sensitive tberraometer -was suspended in the centre of the can, and 

 the air there was maintained at a temperature about half a degree below the 

 maximum temperature which would be attained by the solution in the flask. Tiie 

 time required for complete solution was about 55 seconds, the rotatory motion 

 given to the liquid being- kept up all the time. The moments of maximum tem- 

 perature and complete solution were almost always nearly coincident in each 

 experiment. The total weipfht of the whole apparatus (excluding acid and powder) 

 was 20'5 grammes, and its water equivalent was taken as 3-5 grammes. 



Alloy C contains the metals in their ordinary combining cliemical proportions. 



7. A Platinum Voltmeter, By Professor H. L. Callendar, M.A., F.R.S. 



This is an instrument for measuring electric pressure or current by means of 

 the increase of resistance of a fine platinum wire due to the heating effect of the 

 current passing through it. It is most suitable for use as a voltmeter. Like other 

 hot-wire voltmeters, it is equally available for direct or alternating currents. 

 Unlike those instruments which depend upon increase of length with rise of tem- 

 perature, the fine wire is not subjected to any strain. This makes it possible to use 

 a much finer wire, securing greater delicacy and sensitiveness, and also consuming 

 less power. The change of resistance is very large, being 100 to 200 per cent., and 

 can be measured with great accuracy, whereas the change of length in the expan- 

 sion voltmeter is very minute, and necessitates the use of magnifying gear, whieli 

 is liable to introduce friction or strain. The platinum voltmeter, being simply ii 

 platinum resistance thermometer, constructed of very fine wire, is perfectly fret^ 

 from change of zero. 



The changes of resistance of the fine wire are most conveniently indicated and 

 recorded by means of the automatic recorder which is used for electrical 

 thermometers. This apparatus was exhibited at the conversazione of the Royal 

 Society. It consists essentially of a bridge-wire, forming part of a Wheatstone 

 bridge or potentiometer, iu which the galvanometer contact is automatically main- 

 tained at the balance point by means of a pair of motors controlled by the galvano- 

 meter. Tho recording pen is directly attached to the contact piece, and moves 

 -with it along a straight slide. 



(The apparatus was exhibited in action, together with specimen records, and 

 illustrative lantern-slides.) 



The construction of the voltmeter is simply that of a platinum thermometer, 

 exactly similar to those used for determining tlie temperature cycles of steam in 

 the cylinder of a steam-engine, as described in the Proc. Inst. C. E. vol. cxxxi. 



