188 EXPERIMENTAL PHYSIOLOGY. 



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. 



The lamp used is an ordinary 

 paraffin one provided with a 

 copper chimney with a plane- 

 glass in front and a concave 

 mirror behind. When lighted, 

 the edge of the flame is turned 

 toward the slit. 



(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 swings slowly. 



Fig. 79. Lamp and Scale for 

 Thomson's Galvanometer. 



(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. To Make Non-polarisable Electrodes. One may use the 

 old form as invented by du Bois-Reymond, the simple tube 

 electrodes, or the " brush-electrodes " of v. Fleischl. 



(a.) Use glass tubes about 4 cm. long, and 5 mm. in 

 diameter, tapering somewhat near one end, and see that they 

 are perfectly clean. 



(6.) 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 to plug it, by means of a fresh cut piece of wood or 

 thin glass rod ; allow part of the clay to project beyond the 

 tapered end of the tube (Fig. 80, t, t). 



(c.) By means of a perfectly clean pipette, half fill the 

 remainder of the tube with a saturated neutral solution of 

 zinc sulphate. Make two such electrodes. 



