THE PRODUCTION OF HEAT IN MUSCLE 



247 



met by the difficulty that every junction in the circuit tends to become tl o 

 scat of an electromotive force in consequence of slight changes of tempera- 

 ture due to currents of air, &c. It is therefore advisable to use a plan adopted 

 by Blix, of placing all the apparatus, the muscle included, within the galvano- 

 meter case. The arrangements of such an experiment as employed by A. V. 

 Hill are shown in the diagram (Fig. 73). 



In this instrument the junction of copper with the alloy constantan 

 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, 



Magnet & Mirror Chamber Quartz Fibre 



Mirror 

 Groove 



Constantan Plug 



Muscle 



Electrode 



Broca Magnet 

 ' System 



Copper Coil 



Celluloid Plate 



Electrode 



FIG. 73. 



and into them fit the edges of an outer case 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 connected 

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

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

 hangs down through holes bored in the broad copper coil. The two semi- 

 membranosus 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 



