ELECTRO-MOTIVE PHENOMENA. ^35 



the mirror on the zero of the scale ; adjusting, if necessary, by 

 means of the magnet moved by the milled head on the top of the 

 glass shade (fig. 159, m). 



(c.) Test the electrodes, either by bringing them together or by 

 joining them with a piece of silk thread covered with china-clay 

 paste. After removing all the plugs from the shunt, there ought to 

 be no deflection of the spot of light. If there is none, there is no 

 polarity, and the electrodes are perfect. 



((?.) Ascertain the Direction of Current in Galvanometer. 

 Make a small Smee's battery with a two-ounce bottle. Place in the 

 bottle dilute sulphuric acid (i : 20) and two wires of zinc ( - ) and 

 copper ( + ), with wires soldered to them. Connect them with 

 the galvanometer. Arrange the shunt so that T ^- or T ^Vrr part 

 of the current thus generated goes through the galvanometer. 

 Note the deflection and' its direction. Arrange the N.P. electrodes 

 in the same way, and "observe which is the negative and which 

 the positive pole corresponding to the zinc and copper of the 

 battery. 



(e.) Prepare a Muscle. Dissect out either the sartorius or 

 semi-membranosus of a frog, which consist of parallel fibres, but 

 avoid touching the muscle with the acid skin of the frog. Lay 

 the muscle on a glass plate or block of paraffin under the moist 

 chamber. 



(/.) Keep one plug in the shunt at C, to short-circuit the elec- 

 trodes, and the other plug at --. Cut a fresh transverse section at 

 one end of the muscle, and adjust the point of one electrode exactly 

 over the centre (equator) of the longitudinal surface of the muscle. 

 Apply the other electrode exactly to the centre of the freshly 

 divided transverse surface (fig. 159). 



(g.) Current of Injury. Remove the short-circuiting plug, C, 

 from the shunt, keep one plug in at i, so that y 1 ^ of the total 

 current from the muscle goes through the galvanometer. Note the 

 direction and extent of the deflection. By noting the direction, and 

 from the observation already made (d), one knows that the longi- 

 tudinal surface of the muscle is + , and the transverse section - . 

 Replace the plug-key (C), and allow the needle to come to rest at 

 zero. The deflection was caused by the current of injury, and it 

 flows from the equator or middle of the muscle towards the cut 

 ends. It is also called the demarcation-current. The injured part 

 of a muscle is negative to the uninjured part, and the current in the 

 galvanometer is from the longitudinal ( + ) surface to the injured 

 negative transverse surface. 



(h.) Bring the N.P. electrode on the longitudinal surface nearer 

 to the end of the muscle, and note the diminution of the deflection 

 of the needle. Replace plug C. 



