ANIMAL AND VEGETABLE 235 



ebonite cups, but a good plan is to let a slate or marble 

 slab into the wall and stand the galvanometer upon it. 



At the base of the instrument are two spirit-levels, and 

 the next thing to be done is, by manipulation of the 

 levelling screws, to see that each air-bubble lies exactly in 

 the centre. 



Fig. 124. 



Rising from the top of the case will be seen four 

 terminals and a central brass pillar. Unscrew the 



