232 



PRACTICAL PHYSIOLOGY. 



[XLV. 



"by vibrations, e.g.y on a slate slab fixed into the wall, or on a solid 

 stone pillar fixed in the earth, taking care that no iron is near. 



(h.) Let the galvanometer face wef^t^ ?.e., with the plane of the 

 coils in the magnetic meridian, the magnetic meridian being ascer- 

 tained by means of a magnetic needle. As the galvanometer is a 

 differential one, to convert it into a single one, connect the two 



FlO. 156.— Lamp and Scale for Iliom- 

 Bon's Galvanometer. 



Fio. 155.— Sir William Thomson's Re- 

 flecting Galvanometer, u. Upper, 

 I. Lower coil ; «, «. Levelling screwB ; 

 m. Magnet on a brass support, b. 



FlQ. 157. — Non- 

 Polarisable Elec- 

 trodes. Z. Zincs; 

 K. Cork ; a. Zinc 

 sulphate solu- 

 tion ; t, t. Claj 

 points. 



central binding screws on the ebonite base by means of a copper 



wire. 



(c.) Ey means of the three screws level the galvanometer. 



('/.) Take off the glass cover and steadily raise the small milled 

 head on the top of the upper coils, which frees the mirror, and 

 allows it to swing free. Replace the glass shade. 



(e.) Place the scale (fig. 156) also in the magnetic meridian and 



