220 



PHYSIOLOGY 



In this instrument the junction of copper with the alloy constaiitan constitutes 

 a thermo-electric couple. The magnet and mirror chamber is entirely separated off 

 from the rest of the instrument by the walls of the tube containing the magnet. The 

 grooves are usually filled with plasticine, and into them fit the edges of an outer cage 

 of brass constituting the walls of the muscle chamber. The inside of this case is lined 

 with wet blotting-paper. The copper coil consists of many more turns than are shown 

 in the figure : its ends, AA and BB, are separated by the celluloid plate, and are con- 

 nected by the constantan plug ; the points where the copper meets the constantan 

 constitute the thermo-electric junctions. The tube containing the magnet hangs 



Magnet & Mirror Chamber I Quartz Fibre 



Mirror 

 Groove 



Constantan Plug 



Muscle 



Electrode 



Broca Magnet 

 System 



Copper Coil 



Celluloid Plate 



Electrode 



FIG. 74. 



down through holes bored in the broad copper coil. The two semimembranosus muscles 

 ride astride of the celluloid plate, one in contact with each end of the constantan plug. 

 The small piece of bone at their upper ends which has been left connecting the two muscles 

 is placed exactly on the top of the celluloid plate at x and held in position by a clamp 

 (not shown in the figure). The copper terminals of the coil are coated with celluloid 

 varnish to prevent short circuiting of the thermo-electric currents, and to prevent 

 poisoning of the muscles. Each muscle is in contact with a pair of electrodes, made 

 of fine platinum wire ; the muscle lies over the upper and beneath the lower of these 

 electrodes, as shown in the figure. The tendons at the lower ends of the muscles are 

 tied to silk threads which pass through holes in the base of the instrument. These are 

 then attached to recording levers which write on a drum beneath the table. When 

 all is ready three heavy soft-iron cylinders are placed over the instrument : the latter 

 is screwed to a wooden block which is fixed to a thick iron plate attached to the table. 

 In the cylinders holes are bored to admit and let out after reflexion the light from a 

 Nernst lamp. The lamp, which is about three metres away, shines upon the mirror, 

 and a line in it, after reflexion, is focussed on to the screen, which is also three metres 

 away. The line is brought on to the scale by the small field exerted by a control magnet 

 placed outside the cylinders at a suitable position on the table : its position may be 

 read easily to half a millimetre, and the movement due to a twitch is usually of the order 

 of 80 mm. These soft-iron cylinders cut off entirely all external magnetism sufficient 

 to cause harmful disturbance during an experiment. They lower very largely the 

 strength of the constant external field in which the magnet lies, and leave it chiefly 



