27(3 SEC. 10. ELECTRICITY. 



Mica can be rendered electrical by the least friction, hence its frequent em- 

 ployment us an excellent isolating material, especially on account of the 

 facility with which it can be worked. 



1372. Two Large Condensers, consisting of Leydeii jars, 

 each 400 millimeters high and 200 millimeters in diameter, which 

 .serve as Avell for one machine as the other. Borchardt, Hanover. 



1372a. Adjustable Disc Condenser, which has also been 

 used as a spark micrometer. Sir William Thomson. 



It was in this instrument that the sound produced in an air condenser by a 

 sudden change of potential was first heard. The lower, part of the cell is 

 arranged to hold cups of pumice-stone impregnated with sulphuric acid. 



1372b. Cylindrical Condenser, for measuring capacity in 

 absolute electrostatic units, described iii Messrs. Gribson and Bar- 

 clay's paper in the Transactions of the Royal Society for 1871. 



Sir William Thomson. 



1372c. Condenser for the Holtz Bertsch Electrical 

 Machine. Messrs. Mottersnead $ Co., Manchester. 



IV. APPARATUS FOR PRODUCING AND OBSERVING 

 EFFECTS OF ACCUMULATED ELECTRICITY. 



1373. Apparatus for demonstrating the fundamental laws of 

 electrical and magnetical attraction and repulsion, made according 

 to the instructions of Professor Ed. Hagenbach-BischofF, in Basic. 



G. Lindcr, Basle. 



The ebonite rods are negatively electrified when rubbed with fur, and 

 positively electrified when rubbed with gun cotton. (See Carl, Eepertorium 

 der Experimental Physik, VIII., p. 75.) 



1374. Insulated Pith-Ball Stand, with mahogany arm ; 

 the arm is itself a box in which the pith balls maybe placed when 

 it is not in use. Museum of King George ///., King's College. 



1648. Series of Elder Pith Balls. 



Warmbrunn, Quilitz, and Co., Berlin. 



V. APPARATUS FOR PRODUCING AND OBSERVING 

 EFFECTS OF THE DISCHARGE OF ACCUMULA- 

 TED ELECTRICITY, WITH SPECIMENS OF PER- 

 MANENT RESULTS PRODUCED. 



1375. Photographs of Sparks from a Holtz Machine, in 



cold and in heated air. (See Trans. R. S. Edin., 1874-5.) Taken 

 by an instantaneous process, a quartz lens being employed. 



Professor Tait, Edinburgh. 



