The bearded face of the German grew purple under Welter’s jokingly overbearing manner; but he turned to the instruments and began to explain them. The pneumograph, which the professor first took up, consists of a very thin flexible brass plate suspended by a cord around the neck of the person under examination, and fastened tightly against the chest by a cord circling the body. On the outer surface of this plate are two small, bent levers, connected at one end to the cord around the body of the subject, and at the other end to the surface of a small hollow drum fastened to the plate between the two. As the chest rises and falls in breathing, the levers press more and less upon the surface of the drum; and this varying pressure on the air inside the drum is transmitted from the drum through an airtight tube to a little pencil which it lowers and raises. The pencil, as it rises and falls, touching always a sheet of smoked paper traveling over a cylinder on the recording device, traces a line whose rising strokes represent accurately the drawing of air into the chest and whose falling represents its expulsion.
It was clear to Trant that the professor’s rapid explanation, though plain enough to the psychologists already familiar with the device, was only partly understood by the big men. It had not been explained to them that changes in the breathing so slight as to be imperceptible to the eye would be recorded unmistakably by the moving pencil.
Professor Schmalz turned to the second instrument. This was a plethysmograph, designed to measure the increase or decrease of the size of one finger of a person under examination as the blood supply to that finger becomes greater or less. It consists primarily of a small cylinder so constructed that it can be fitted over the finger and made airtight. Increase or decrease of the size of the finger then increases or decreases the air pressure inside the cylinder. These changes in the air pressure are transmitted through an airtight tube to a delicate piston which moves a pencil and makes a line upon the record sheet just under that made by the pneumograph. The upward or downward trend of this line shows the increase or decrease of the blood supply, while the smaller vibrations up and down record the pulse beat in the finger.
There is still a third pencil touching the record sheet above the other two and wired electrically to a key like that of a telegraph instrument fastened to the table. When this key is in its normal position this pencil makes simply a straight line upon the sheet; but instantly when the key is pressed down, the line breaks downward also.
This third instrument is used merely to record on the sheet, by the change in the line, the point at which the object that arouses sensation or emotion is displayed to the person undergoing examination.
The instant’s silence which followed Schmalz’s rapid explanation was broken by one of Welter’s companions with the query:
“Well, what’s the use of all this stuff, any way?”
“Ach!” said Schmalz, bluntly, “it is interesting, curious! I will show you.”
“Will one of you gentlemen,” said Trant, quickly, “permit us to make use of him in the demonstration?”
“Try it, Jim,” Welter laughed, noisily.