IX. RESISTANCE. 341 



tion of water be shown, but the phenomena of heating and ignition, e.g., of a 

 steel spring. For such experiments 12 Bunsen pairs are necessary, connected 

 in series. 



1486b. Silver Voltameter, with platinum vessel. 



Prof. Beetz, Munchen. 

 A convenient modification of the Pog^endorff voltameter. 



148 6c. Mercury Voltameter. 



The Physical Science Laboratory of the Technological 

 Institute, St. Petersburg (Russia). 



This apparatus is intended for measuring the strength of currents by the 

 reduction of mercury. It consists of two electrolytic glass bowls joined by a 

 tube, and an apparatus for measuring the reduced mercury volumetrically. 

 A more detailed description, with drawing and directions for use, accompany 

 the instrument. 



1732b. Electrotype Apparatus, with stand, and Poggen- 

 doi-ff's silver voltameter. Prof. Hittorf, Munster. 



The vessels serve in investigating the processes by which the electrolytes 

 in aqueous, alcoholic, or other solutions afford passage to the electric current. 

 They enable the changes occurring in the electrodes to be kept distinct 

 and fully determined by quantitative chemical analysis. In soluble elec- 

 trolytes the ions can be certainly determined, and so the primary decom- 

 positions distinguished from the secondary. Further, the apparatus shows 

 the ratio of the velocities with which the two ions move in opposite directions. 

 {See Poggendorf s Annaleii, Bd. 89, 96, 103, and 106.) 



IX. APPARATUS FOR MEASURING ELECTRICAL 

 RESISTANCE AND CAPACITY. 



1487. Wheatstone's Bridge of the simplest construction^ 

 especially used for conductivity and low resistance tests. The 

 bridge is provided with one pair of equal resistances and one 

 standard resistance. Elliott Brothers. 



1488. Thomson's Circular Sliding Resistance, of great 

 Importance in cable tests in connexion with the quadrant elec- 

 trometer for measuring resistance, tension, or potential of cables 

 and batteries, and for detecting faults in cables. 



Elliott Brothers. 



1488a. Megohm or 1 million Ohms resistance, valuable 

 for measuring very high resistances, for determining the constants 

 of galvanometers, <fcc. Elliott Brothers. 



1487a. Revolving Coil used in the determination of the 

 " British Association Unit of Electrical Resistance,' 9 



1863-4. (The property of the British Association.) 



Prof. Clerk Maxwell. 



