ELECTRICAL ANALOGY 147 



coils is converted partly into magnetic potential energy and partly into 

 heat energy (Joule's heat) which is dissipated. The magnetic potential 

 energy is converted partly into the kinetic form (rotation) and is partly 

 dissipated as heat. The proportion of the incident energy which manifests 

 itself as " free " and as " bound " depends on the length of time during 

 which the current passes. A short excitation leads to the development 

 of power in preference to heat while the reverse is the result of a prolonged 

 excitation. Similarly, with muscle a single twitch develops very little 

 initial heat while an increasing fraction of the potential energy appears as 

 heat as the mechanical response is prolonged. 



(ii) Maintenance. In order to maintain a constant magnetic field a 

 constant current has to be maintained in the coils of the armatures. The 

 coils heat up. This is analogous to a tetanic contraction. 



(iii) Relaxation. The induced currents, set up in the coils and in the 

 neighbouring conductors by breaking the circuit, are converted into heat, 

 i.e. the evolution of heat continues until the whole of the magnetic energy 

 of the field has been dissipated as heat. Similarly in the muscle when the 

 stimulus ends, the heat production does not stop at once ; it continues 

 until all the elastic potential energy of the muscle has disappeared. 



(iv) Recovery. None of the above processes require the immediate 

 presence of oxygen. In order to restore the electrical system to its former 

 level, i.e. to recharge the accumulator, some mechanism has to be employed 

 to convert kinetic into electrical energy. The normal process implies 

 oxidation an engine is started, oxygen and fuel are consumed, a dynamo 

 is driven and current produced. Similarly oxygen is necessary during the 

 recovery phase of muscular contraction. 



(v) Rest. Accumulators " run down " when not in use, and liberate 

 a certain small amount of heat. To keep them in condition they must 

 have periodical additions to their material and energy contents. This 

 recharging involves heat production and oxidation. In the same way, 

 muscle at rest (in the absence of oxygen) liberates heat (and lactic acid). 

 In the presence of oxygen this liberation is made good by oxidation. That 

 is, muscle normally is prevented from " running down " by " continuous 

 recharging " with, of course, a steady evolution of heat. 



The foregoing notes are taken materially and almost verbally from an 

 article by Prof. A. V. Hill and W. Hartree in the Journal of Physiology, 

 Vol. LIV., p. 84. The student will, of course, bear in mind that no theory 

 of muscular contraction is implied. A physical phenomenon which bears 

 a very close resemblance to certain phases of the mechanical response 

 of muscle to stimulation is given as suggestive of a type of mechanism. 

 But, as has been said, " Resemblance is no proof analogies can never 

 replace analysis." 



