PSYCHOLOGICAL PROGRAM AND TECHNIQUE. 177 



ble arm, but even with this improvement it was quite unsatisfactory 

 as a measurement. According to the best determination, the resist- 

 ance of the tissue of the finger-tips when immersed to a depth of 2 cm. 

 in salt solution is in the neighborhood of 4,000 to 5,000 ohms. 



An effort was made to include a large resistance in series with the 

 secondary circuit so that the tissue resistance of the fingers would be but 

 a small fractional part and could be assumed without actual measure- 

 ment. With this sort of a change it is necessary to use a stronger cur- 

 rent in the primary coil of the induction apparatus. The sparking at 

 the breaking of the primary circuit becomes very objectionable under 

 this condition and a source of considerable error. Even with the 

 larger coils it is not possible to include 100,000 ohms and still have 

 sufficient strength of shock in the secondary circuit for purposes of 

 stimulation. Larger induction coils were experimented with, such as 

 those used in commercial transformers. One of these proved somewhat 

 more satisfactory in that a large resistance, such as 200,000 ohms, could 

 be included in a secondary circuit with the fingers and still enough 

 strength of shock be provided for purposes of stimulation, without the 

 necessity of increasing the current in the primary circuit to an objec- 

 tionable amperage. Since the primary and secondary windings had a 

 fixed relation to each other in this coil, the changing of the strength of 

 the induced current would necessitate a change in the current at the 

 primary. Theoretically, it seemed that this would also change the 

 wave-form of the induced current. 



An investigation was carried out to determine if the wave-form 

 changed materially within the range of change in strength of primary 

 current, which was desirable for purposes of sensory-threshold determi- 

 nation. In the course of this investigation it was observed that the 

 change in wave-form of the induced current was considerable when the 

 primary current was changed through that range which was necessary 

 for experimental purposes. Another factor also revealed itself, i. e., 

 that the wave-form of the induced current was varied by the amount of 

 resistance in series with the secondary. The larger the resistance 

 placed in series, the steeper was the rise of the current curve and the 

 less the time to the maximum and steeper the fall. In the light of these 

 facts, many of which are well known, it seemed highly desirable to dis- 

 continue the use of induced current and to use direct-current stimula- 

 tion if possible. While most threshold measurements are undoubtedly 

 subject to a certain percentage of error, this does not warrant the use of 

 apparatus with variable factors for which corrections can not be made. 



Many arrangements for using direct-current stimulation were tried; 

 finally a simple one was adopted. This is shown in schematic diagram 

 in figure 43. The apparatus consists of six main items: a drop wire D; 

 a voltmeter V; a non-inductive resistance R; a pendulum, indicated by 

 the arrow A, for breaking the circuit at points denoted by S and S'; 



