C. GENEKAL PHYSICS. 193 



the eminent metallurgist, habitually employs his pyrometer 

 for the determination of the temperatures employed in vari- 

 ous operations of the blast furnace. 



The third part of Mr. Siemens's paper is a highly suggestive 

 and valuable memoir on a simple method of measuring elec- 

 trical resistances. He states that although a Wheatstone 

 balance furnishes electricians with the means of measuring 

 the resistances of electrical circuits with great accuracy, yet 

 its application is, in many cases, rendered difficult on account 

 of the delicacy of the apparatus and of extraneous disturbing 

 causes. As a portable instrument, and one especially appli- 

 cable to observations on shipboard and in exploring expedi- 

 tions, he proposes what he calls a differential voltameter, 

 which consists of two similar narrow closed tubes fixed ver- 

 tically to a wooden frame, with a divided scale behind them, 

 and whose lower ends, being enlarged somewhat, are fitted 

 with wooden stoppers saturated with paraffin, and pene- 

 trated by platinum wires. Diluted sulphuric acid is ad- 

 mitted into these tubes, and kept at a proper height in each 

 by a very simple device, and the evolution of gas that occurs, 

 when a current passes from the electrodes, affords the meas- 

 ure of the strength of the current. By means of a commu- 

 tator the current from the battery is easily reversed every 

 few seconds, preventing polarization of the electrodes. By 

 introducing the resistance of the voltameter, and the un- 

 known resistance x, first on one, and then on the other side 

 of the arrangement, the observations, by a simple arithmet- 

 ical process, give the exact value of the unknown resistance. 

 The measurement of the quantity of decomposed gases serves 

 merely to determine the relative intensity of the currents 

 which flow in the respective positions of the commutator. 

 He states that, having measured numerous resistances by 

 this instrument, and compared the results witli measure- 

 ments obtained by a very perfect Wheatstone bridge ar- 

 rangement, he finds that it may be relied upon within a 

 half per cent, of error of observation. The instrument es- 

 pecially recommends itself for use on board of vessels, as 

 not being in the slightest degree influenced either by the 

 motion of the vessel or by the magnetic influence of mov- 

 ing masses of iron. One of its intrinsical advantages is that 

 it gives the resistance to be measured in units of work done, 



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