54* 



REFLECTING GALVANOMETER AND SHUNT. 



small milled head. When the milled head is raised, the system of needles swings freely. 

 The coils are protected by a glass shade, and the whole stands on a vulcanite base, which 

 is levelled by three screws (s, a). On a brass rod (b) is a feeble magnet (m), which is used to 

 give an artificial meridian. The magnet (m) can be raised or lowered by means of a nulled 

 head.] 



[Lamp and Scale. When the instrument is to be used, place it so that the coils face east and 

 west. At 3 feet distant from the front of the galvanometer, facing west, is placed the lamp and 

 scale (fig. 38S). There is a small vertical slit in front of the lamp, and the image of this slit is 

 projected on the mirror attached to the upper needles, and by it 

 is reflected on to the paper scale fixed just above the slit. The 

 spot of light is foeussed at zero by means of the magnet, 7n. 

 The needles are most sensitive when the oscillations occur 

 slowly. The sensitiveness of the needles can be regulated by 

 means of the magnet. In every case the instrument must be 

 quite level, and for this purpose there is a small spirit-level in 

 the base of the galvanometer.] 



[Shunt. As the galvanometer is very delicate, it is con- 

 venient to have a shunt to regulate to a certain extent the 

 amount of electricity transmitted through the galvanometer. 

 The shunt (fig. 389) consists of a brass box containing coils of 

 German silver wire, and is constructed on the same principle as 

 resistance coils or the rheocord ( 326). On the upper surface 

 of the box are several plates of brass separated from each other, 

 like those of the rheocord, but which can be united by brass 

 plugs. The two wires coming from the electrodes are connected 

 Lamp and scale for Thomson's w j t g tne two binding screws, and from the latter two wires are 

 galvanometer. i ( , j ^ ^ e outer two binding screws of the galvanometer. By 



placing a plug between the brass plates attached to the two binding screws in the figure, the 

 current is short-circuited. On removing both plugs, the whole of the current must pass 

 through the galvanometer. If one plug be placed between the central disc of brass and the 

 plate marked \, (the other being left out), then ^th of the current goes through the galvano- 

 meter and T \ths to the electrodes. If the plug be placed as shown in 

 the figure opposite v \, then T T th part of the current goes to the gal- 

 vanometer, while ^ths are short-circuited. If the plug be placed 

 opposite r ,lv, only W^th part goes through the galvanometer.] 



Internal Polarisation of Moist Bodies. Nerves and muscular fibres, 

 the juicy parts of vegetables and animals, fibrin, and other similar bodies 

 possessing a porous structure filled with fluid, exhibit the phenomena of 

 polarisation when subjected to strong currents a condition termed 

 internal polarisation of moist bodies by du Bois-Reymond. It is assumed 

 that the solid parts in the interior of these bodies which are better 

 conductors, produce electrolysis of the adjoining fluid, just like metals 

 in contact with fluid. The ions produced by the decomposition of the 

 internal fluids give rise to differences of potential, and thus cause in- 

 ternal polarisation ( 333). 



Cataphoric Action. If the two electrodes from a galvanic battery 

 be placed in the two compartments of a fluid, separated from each other 

 by a porous septum, we observe that the fluid particles pass in the 

 direction of the galvanic current, from the + to the - pole, so that 

 after some time, the fluid in the one half of the vessel increases, while it 

 diminishes in the other. The phenomena of direct transference were 

 called by du Bois-Reymond the catapJwric action of the constant current. The introduction of 

 dissolved substances through the skin by means of a constant current depends upon this action 

 ( 290), and so does the so-called Porret's phenomenon in living muscle ( 293, I., b). 



External Secondary Resistance. This condition also depends on cataphoric action. If the 

 cooper electrodes of a constant battery be placed in a vessel filled with a solution of cupric 

 sulphate, and from each electrode there project a cushion saturated with this fluid, then, on 

 placing a piece of muscle, cartilage, vegetable tissue, or even a prismatic strip of coagulated 

 albumin across these cushions, we observe that, very soon after the circuit is closed, tnere is 

 a considerable variation of the current. If the direction of the current be reversed, it first 

 becomes stronger, but afterwards diminishes. By constantly altering the direction of the 

 current we cause the same changes in the intensity. If a prismatic strip of coagulated albumin 

 be used for the experiment, we observe that, simultaneously with the enfeeblement of the 

 current in the neighbourhood of the + pole, the albumin loses water and becomes more 

 shrivelled, while at the - pole the albumin is swollen up and contains more water. If the 

 direction of the current be altered, the phenomena are also changed. The shrivelling and 

 removal of water in the albumin at the positive pole must be the cause of the resistance in the 



Fig. 389. 

 Shunt for galvano- 

 meter. 



