248 



PHYSIOLOGY 



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 disturbances during an 

 experiment. They lower very largely the strength of the constant external 

 field in which the magnet lies, and leave it chiefly supported in any position 

 by the quartz fibre. Thus all the movements set up in the magnet by the 



1.1 



FIG. 74. Arrangement of apparatus for measuring small differences of 

 temperature. 



thermo-electric currents are working against little more than the torsion 

 of a quartz fibre only 6 p thick. This explains the great sensitivity of the 

 instrument. 



A second method depends on the fact that rise of temperature increases the 

 resistance of a wire to the passage of an electric current. A current detector 

 consists of a small grid of fine platinum wire which is placed against the muscle 

 between two muscles. This grid is then made one limb of a Wheatstone's 

 bridge (Fig. 74). A small current is passed through the circuit, and the resistances 

 are so adjusted that no current flows through the galvanometer. Any alteration 

 in temperature of the grid will alter the balance of the resistance and will cause 

 a current to flow through the galvanometer in a direction which will vary 

 according as the resistance in the grid is increased or diminished. It is possible 

 to calibrate the arrangement so that a deflection of the galvanometer over one 

 degree will correspond to a certain fraction of a degree of difference in tem- 

 perature of the grid. This method is employed in Callender's recording thermo- 

 meters, and has been made by Gamgee the basis of an arrangement for the 

 continuous record of the temperature of the human body. 



The discovery of exact means of measuring the heat production 

 during contraction was naturally utilised to determine the relation 

 between the heat produced and the work done under varying con- 

 ditions. In the muscle, as in a steam-engine, we have a conversion 

 of potential energy stored up in carbon compounds into kinetic 

 energy, which may appear as work and heat. In the engine there 



