OP ARTS AND SCIENCES. 455 



have deteriorated any, although it has been in constant use since 

 November 15, 1884. 



An extra cell was made whose electromotive force was 1.04 volts. 

 It has been tested occasionally since, and found to remain constant. 



A distilled water cell was completely covered with paraffine. Al- 

 though it has remained nearly constant, some of the water has been 

 lost, and the zinc is now coated with a thick deposit. 



At the same time these cells were made, we made another in the 

 following manner. A copper cylinder six centimeters long and one 

 centimeter in diameter was filled with plaster of Paris mixed with a 

 solution of zinc chloride containing a small per cent of sodium chloride. 

 A zinc wire was placed in this, and the plaster allowed to dry. This 

 cell has an electromotive force of .80 volt, and an internal resistance 

 much less than a Beetz cell. It has remained constant up to date. 



A Beetz battery can be made of compact size, and imbedded in 

 paraffine or some solid insulating substance, and made portable so as 

 to be used in connection with a portable electrometer. 



This is decidedly the best kind of a battery to use in electrostatic 

 measurements. It has the advantage over the water battery of being 

 cheaper, of being smaller and more convenient to move, and it re- 

 quires little attention after it is made. 



The Collector. 



The water-dropping collector proposed and used by Sir William 

 Thomson* was employed at first. It consists of an insulated metallic 

 vessel filled with water and connected to the electrometer. AVater 

 drops from the nozzle of the vessel, and reduces it to the potential of 

 the air at the point where the stream ceases to be continuous. 



A more convenient arrangement of the same principle is to allow 

 the water to drop first on an insulated metallic plate connected with 

 the electrometer, and then to the ground. The vessel containing the 

 water need not be metallic nor insulated. Water dropping from the 

 plate reduces it to the potential of the air, in the same manner that 

 the nozzle of the vessel was reduced. The metal plate used was of 

 brass 10 cm. by 15 cm. It was placed from five to ten centimeters 

 below the nozzle. Different-sized plates of copper and of brass were 

 used without appreciably affecting the results. With zinc and other 

 plates there was a slight contact electricity between the plate and 

 the brass quadrants of the electrometer. The plate has the double 



* Papers on Elect, and Mag., § 202. 



