XXV.] GALVANIC BATTERIES AND GALVANOSCOPE. 159 



be placed in a draught cham 1 er to prevent the nitrous fumes 

 from affecting the experimenter. 



5. Bichromate Cell (fig. 82). This 

 consists of a glass bottle containing 

 one zinc and tw3 carbon plates im- 

 mersed in the following mixture : 

 Dissolve i part of potassic bichromate 

 in 8 parts of water, and add I 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 in- 

 tensity. 



6. Leclanche Cell. The positive 

 plate is zinc in ammonium chloride 

 solution (Zinc -pole). The negative 

 plate is carbon with manganese 

 dioxide in the same solution (Car- 

 bon + pole). 



Other forms of batteries are 

 used, but the foregoing are suffi- 

 cient for the purposes of these 

 exercises. 



7. The Galvanoscope or De- 

 tector. 



(a.) Charge a Daniell's cell 

 and attach a copper wire to 

 the negative pole (zinc), and 



another to the positive pole (copper). On bringing the free ends 

 of the two wires together the circuit is made, and a current of 

 continuous, galvanic, or voltaic electricity 

 circulates outside the battery from the + 

 to the - pole. Prove the existence of this 

 current by its effect on a magnetic needle. 



(/;.) Use a vertical galvanoscope or de- 

 tector (fig. 83^, in which the magnetic need e 

 is so loaded as to rest in a vertical position. 

 A needle attached to this moves over a 

 semicircle graduated into degrees. Con- 

 nect the wires from the + and - poles of the 

 Daniell's battery with the binding screws of 

 this instrument, and note that when the 

 circuit is made the needle is deflected from 

 its vertical into a more or less horizontal position, but the angle 



FlG. 82. Bichromate Cell. A. The glass 

 vessel ; K, K. Carbon ; Z . Zinc ; D, E. 

 Binding screws for the wires; B. Rod to 

 raise or depress the zinc in the fluid ; C. 



Screw to fix B. 



