CHAPTER 8 



The Homeostat 



8/1. The ultrastable system is much richer in interesting pro- 

 perties than might at first be suspected. Some of these pro- 

 perties are of special interest to the physiologist and the psycho- 

 logist, but they have to be suitably displayed before their physio- 

 logical and psychological applications can be perceived. For 

 their display, a machine was built according to the definition of 

 the ultrastable system. What it is, and how it behaves, are the 

 subjects of this chapter.* 



8/2. The Homeostat (Figure 8/2/1) consists of four units, each of 

 which carries on top a pivoted magnet (Figure 8/2/2, M in 

 Figure 8/2/3). The angular deviations of the four magnets from 

 the central positions provide the four main variables. 



Its construction will be described in stages. Each unit emits 

 a D.C. output proportional to the deviation of its magnet from 

 the central position. The output is controlled in the following 

 way. In front of each magnet is a trough of water; electrodes 

 at each end provide a potential gradient. The magnet carries 

 a wire which dips into the water, picks up a potential depending 

 on the position of the magnet, and sends it to the grid of the 

 triode. J provides the anode-potential at 150 V., while H is at 

 180 V. ; so E carries a constant current. If the grid-potential allows 

 just this current to pass through the valve, then no current will flow 

 through the output. But if the valve passes more, or less, current 

 than this, the output circuit will carry the difference in one direc- 

 tion or the other. So after E is adjusted, the output is approxi- 

 mately proportional to M's deviation from its central position. j* 



* It was given the name of ' Homeostat ' for convenience of reference, and 

 the noun seems to be acceptable. The derivatives ' homeostatic ' and 

 1 homeostatically ', however, are unfortunate, for they suggest reference to 

 the machine, whereas priority demands that they be used only as derivatives 

 of Cannon's ' homeostasis '. 



t Following the original machine in principle, Mr. Earl J. Kletsky, at the 

 Technische Hogeschool, Delft, Holland, has designed and built a form that 

 replaces the magnet, coils, vane and water by Kirchhoff adding circuits and 

 capacitors. 



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