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AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



electric current will flow from the point where the tension is greater to that where it is less. 

 The presence, direction of flow, and strength of an electric current can be detected by an 

 instrument called a galvanometer. If any two parts of a muscle or nerve, as e, e, Figure 

 58, be connected by suitable conductors with the coils, c, c, of a galvanometer, and if there 

 be a difference in the electric potential of the two parts examined, an electric current will 

 be indicated by the instrument. In such tests all extra sources of electricity are to be 

 avoided, therefore the electrodes applied to the muscle must be non-polarizable. 



The Galvanometer An ordinary form of galvanometer consists of a magnet suspended 

 by an exceedingly delicate fibre of silk, or quartz, and one or more coils, composed of many 

 windings of pure copper wire, placed vertically near the magnet and in the plane of the mag- 

 netic meridian. If an electric current be allowed to flow through the wire, it influences the 

 magnetic field about it, and, if the coils be close to the suspended magnet, causes the 

 magnet to deviate from the plane of the magnetic meridian in one or the other direction, 

 according to the direction of the flow of the current. In the more delicate instruments the 

 influence of the earth's magnetism is lessened by the use of two magnets of as nearly as pos- 

 sible the same strength, placed so as to point in opposite directions, and fastened at the 

 extremities of a light rod. As each magnet tends to point toward the north, they mutually 

 oppose each other, and therefore the effect of the earth's magnetism is partly compensated. 

 Still another magnet may be brought near this "astatic" combination, and by opposing the 

 action of the earth's magnetism make the arrangement even more delicate. In the Thomp- 

 son galvanometer, the rod connecting the needles bears a slightly concave mirror, from 

 which a beam of light can be reflected on a scale. Or a scale may be placed so that its 

 image falls on the mirror, and the slightest movement of the magnet may be read in the 

 mirror by a telescope. 



The galvanometer is very sensitive to the presence of electric currents. Another appa- 

 ratus which is even more responsive to changes in electric potential of short duration is 

 the capillary electrometer. 



The capillary electrometer (Fig. 59) consists of a glass tube (a) drawn out to form 

 a very fine capillary, the end of which dips into a glass cup with parallel sides (/) contain- 

 ing a 10 per cent, solution of sulphuric acid. The 

 upper part of the tube is connected by a thick- 

 walled rubber tube with a pressure-bulb containing 

 mercury (c). As the pressure-bulb is raised, the 

 mercury is driven into the capillary, the flow being 

 opposed by the capillary resistance. By a suffi- 

 ciently great pressure, mercury may be driven to the 

 extremity of the capillary and all the air expelled. 

 When the pressure is relieved the mercury rises 

 again in the tube, drawing the sulphuric acid after 

 it. The column of mercury will come to rest at a 

 point where the pressure and the capillary force just 

 balance. Seen through the microscope (e), the end 

 of the column of mercury, where it is in contact 

 with the sulphuric acid appears as a convex menis- 

 cus (d). Any alteration of the surface tension of 

 the meniscus causes the mercury to move with 

 great rapidity in one direction or the other along 



FIG. 59.-Schema of capillary electrometer. the ^ 5 R " d a V6ry sli ^ ht diflfe rence of electric 



potential suffices to cause a change in surface ten- 

 sion of the mercury-sulphuric acid meniscus. A platinum wire fused into the glass tube 

 (a), and another dipped into a little mercury at the bottom of the cup holding the acid, 

 permit the mercury in the capillary and the acid to be connected with the body the elec- 

 tric condition of which is to be examined. If the mercury and acid be connected with 

 two points of different electric potential, as g and h of muscle M, the mercury will instantly 



