GALVANIC BATTERIES AND GALVANOSCOPE. 



131 



to be amalgamated. Dip the zinc in 10 per cent, sulphuric acid 

 until effervescence commences. Lift it out and place it on a 

 shallow porcelain plate. Pour some mercury on the zinc, and 

 with a piece of cloth rub the 

 mercury well over the zinc. 

 Dip the zinc in the acid again, 

 and then scrub the surface 

 with a rag under a stream of 

 water from the tap. Collect 

 all the surplus mercury and 

 place it in the bottle labelled 

 "Amalgamation Mixture." 

 Take care that none of the 

 mercury gets into the soil 

 pipe. A very convenient 

 method is to dip the zinc into 

 a glass tube with thick walls 

 containing mercury and sul- 

 phuric acid. For convenience 

 the tube is fixed in a block of 

 wood. 



3. Grove's Cell- (Fig 34) 

 consists of an outer glazed 

 earthenware glass or ebonite 

 vessel containing a roll of 

 amalgamated zinc and dilute 

 (10 per cent.) sulphuric acid. 



In the inner porous cell is placed platinum foil with strong 

 nitric acid. The platinum and zinc are provided with binding 

 screws. The platinum is the + positive pole or anode, the zinc 

 the - negative pole or cathode. 



For physiological purposes, the small Grove's cells about 7 

 cm. in diameter and 5 cm. in height, are very convenient. When 

 in use the battery ought to be placed in a draught chamber to 

 prevent the nitrous fumes from affecting the experimenter. 



4. Bichromate Cell (Fig. 35). This consists of a glass bottle 

 containing one zinc and two carbon plates immersed in the fol- 

 lowing mixture : Dissolve 1 part of potassic bichromate in 8 

 parts of water and add 1 part of sulphuric acid. The zinc is 

 attached to a rod, which can be raised when it is desired to stop 

 the action of the battery. This cell is convenient enough when 

 it is not necessary to use a current of perfectly constant intensity. 



Fig. 34. Large Grove's Element. 



