178 VITALITY AND EFFICIENCY WITH RESTRICTED DIET. 



a control switch C, which is also a pole-changing switch; and non- 

 polarizable electrodes E. The switches S and *S' are arranged to be 

 struck open by a pendulum moving from left to right. When struck open 

 they remain open. It is therefore evident from the diagram that with 

 the switches closed, as shown in the figure, the current from the drop- 

 wire has parallel paths P and P'. Since the fingers F of the subject are 

 in series with P', and the tissue of the fingers has a resistance of about 

 5,000 ohms, the current thus goes through P, where the resistance is 

 negligible as long as S is closed. When S is struck open the current is 

 established in P'. This amounts to the ''make" of the circuit. The 

 duration of the shock is regulated by the distance between S and S\ 

 When S' is opened the shock is ended. This provides a shock which 

 theoretically should have as nearly as is possible a curve showing an 

 instantaneous rise, a square top, and an instantaneous fall. The 

 height of the shock is a function 

 of the voltage. The amount of 

 electrical energy actually de- 

 livered at the fingers need not 

 concern us here, as our results 

 are comparative. The thresh- 

 old values are expressed in 

 terms of voltage, read at the 

 voltmeter V, which had an in- 

 ternal resistance of 33,894 ohms 

 at 21° C. 



In any such arrangement of 

 apparatus the device used to 

 make and break the circuit and 

 to determine the length of the 

 shock is of critical importance. 



The particular device constructed at the Nutrition Laboratory for 

 this purpose was a modification of a similar instrument designed 

 by the late Keith Lucas and used by him in his extraordinary 

 researches on muscle and nerve excitation.^ According to illustrations 

 and description, in the case of the Lucas pendulum the contact key had 

 to be set and the pendulum adjusted and released by hand for each 

 stimulation delivered to the nerve or muscle preparation. It was 

 necessary to arrange for these factors to be automatically taken care of 

 in our instrument, the general plan of which is shown in figure 44. 

 Perpendicular to a heavy wooden base B, a short stud T was rigidly 

 mounted. About this an eccentric weight E was mounted on ball 



^ Lucas, Joum. Physiol., 1908, 37, p. 459. With this pendulum Lucas was able to secure elec- 

 trical currents of any duration between 0.0001 and 0.12 second. He gives drawings showing 

 front and side elevations of the instnunent and others for detail of the contact keys. The Lucas 

 pendulum is figured in the catalog of the Cambridge Scientific Instrument Co., England, and is 

 listed as one of their regular pieces of physiological apparatus. 



Fig. 43. — Schematic diagram of circuit for 

 measuring the electrical threshold. 



D, drop wire; V, voltmeter of standard make; R, 

 200,000 ohms resistance; C, control switch; 

 E, non-polarizable electrodes; F, fingers of 

 subject to be stimulated; S and <S', switches 

 to be opened in turn by a swiftly moving 

 pendulum represented by the arrow. A; 

 P and P', parallel paths for current when 

 S and S' are closed. 



