SENSORY FARADIC THRESHOLD. 137 



tinct, they may on occasion flash out intermittently. Furthermore, the 

 throb of the pulse and slight muscle-twitches often appear to concen- 

 trated attention. Geissler 1 found pulse sensations to interfere with 

 minimal-weight sensations. There may thus be some purely physio- 

 logical grounds for the errors which occur under higher doses of alcohol, 

 especially when, as our observations in Chapter VIII show, this is 

 accompanied by an accelerated pulse. 



In general, we may say that a thoroughgoing psychological exploita- 

 tion of the Martin-threshold measurements will probably take into 

 account fatigability, differences between the threshold to increasing 

 stimuli and the threshold to decreasing stimuli, the number and dis- 

 tribution of errors, as well as actual changes in the apparent threshold 

 level. Unless changes in the skin-resistance are considerable, we 

 believe it will be more profitable psychologically to neglect the absolute 

 (3 value, after it is once determined for a given subject and day, and to 

 concentrate attention on a statistical treatment of the simplest thresh- 

 old measurements at skin resistance (Martin Z units). /3 values and Z 

 values are commonly parallel in any event. Concentration on Z meas- 

 urements will consequently not impair the relative significance of the 

 results, while it may give important indications of varying subjective 

 conditions. 



APPARATUS AND TECHNIQUE. 



The general arrangement of apparatus for the sensory threshold to 

 Faradic stimulation (Martin measurements) is seen in figure 14, 

 page 95. Inductorium, mil-ammeter, and resistance boxes are seen to 

 occupy the lower right-hand corner of the main apparatus table. 

 KI indicates the Kronecker inductorium, which was calibrated for 

 the Nutrition Laboratory by Professor Martin. The Martin key for 

 breaking the primary circuit under a column of mercury is not shown 

 in the diagram. In our early experiments, it was operated by an 

 assistant in an adjoining room. In all the data which are reported in 

 this paper subsequent to February 1, an electrically operated key of 

 similar construction was used. A simple contact device held in the 

 operator's hand caused the key to make and break the primary circuit 

 of the coil. A indicates the mil-ammeter. It was continuously in the 

 primary circuit, and served to indicate not only any accidental change 

 in the amount of primary current, but also the exact moment of each 

 stimulation. R 6 is a non-inductively wound resistance-box, ranging 

 from 10,000 to 100,000 ohms resistance. This resistance served to 

 introduce a known resistance of 20,000, 30,000, and 40,000 ohms 

 respectively into the secondary circuit. By use of the double switch 

 at the left, this same box also served as a standard resistance for meas- 

 uring the skin-resistance by the Kohlrausch method. The alternating 

 current for measuring the skin-resistance was furnished by a Porter 

 inductorium which is not shown in the diagram. Connections for 



Geissler, Am. Journ. Psychol., 1907, 18, p. 309. 



