XLV.] ELECTRO-MOTIVE PHENOMENA. 233 



I metre from the mirror, taking care that it is at the proper height. 

 Instead of a slit in the scale, it is better to fix in it a thin wire, and 

 by means of a lens of short focal distance to bring the image of the 

 wire to a focus in the middle of the illuminated disc of light 

 reflected from the mirror upon the scale. 



(a'.) Light the paraffin lamp, place the edge of the flame towards 

 the slit, darken the room, and see that the centre of the scale, its 

 zero, the slit in the scale, the flame of the lamp, and the centre of 

 the mirror, are all in the same vertical plane, so that a good light 

 is thrown on the mirror in order to obtain a good image on the 

 scale. 



{g.) Make the needle all but astatic by means of the magnet 

 attached to the bar above the instrument. The needle is most 

 sensitive when it smngs slowly. 



(h.) Test the sensitiveness of the galvanometer by applying the 

 tips of two moist fingers to the two outer binding screws of the 

 instrument, when at once the beam of light passes off the scale. 



2. Non-Pol arisable Electrodes.— One may use the old form 

 of Du Bois-Reymond, the simple tube electrodes, or the "brush 

 electrodes « of Y. Fleischl (fig. 160). 



(A.) (a.) Use glass tubes about 3 cm. long and 5 mm. in diameter, 

 tapering somewhat near one end, and see that they are perfectly 

 clean. 



(h.) Plug the tapered end of the glass tube with a plug of china 

 clay, made by mixing kaolin into a paste with normal saline. 

 Push the clay into the lower third or thereby of the tube ; plug 

 the latter, using a fresh-cut piece of wood or thin glass rod to do 

 so ; allow part of the clay to project beyond the tapered end of the 

 tube (fig. 157, ^, t). 



(c.) With a clean pipette half fill the remainder of the tube with 

 a saturated neutral solution of zinc sulphate. Make two such 

 electrodes. 



{d.) Into each tube introduce a well-amalgamated piece of zinc 

 wire with a thin copper wire soldered to its upper end (Z, Z), fix 

 the electrodes in suitable holders in a moist chamber, and attach 

 the wires of the zincs to the binding screws on the stage of the 

 moist chamber. The zinc should not touch the clay. 



(B.) Some prefer a [J -shaped glass tube held in a suitable 

 holder attached to a vulcanite rod in the moist chamber 

 (B. Sanderson's pattern). The tube contains a saturated solution of 

 zinc sulphate as before. Into one limb of the tube is placed the 

 rod of amalgamated zinc. In the other free Hmb is placed a 

 straight tube with a slight flange at its upper end filled with kaolin 

 moistened with normal saline, the kaolin projecting as a cap above 



