so that each one will bring the bridge into balance in one of the ten temperature 

 ranges covered by the instrument. R4 is made up of the resistance of two thermis- 

 tor beads in parallel, Rtr in series with K4S (field resistor). The circuit is com- 

 pleted by the resistor Rg in parallel with the leads to the recorder. Ris and i?5 

 compensate for the difference in voltage output at high and low temperatures, 

 thus making each subrange approximately the same, namely, 7.0° to 7.5 °F, from 

 28.0° to 90°F; each subrange overlaps the next by about 0.5 °F. 



Figure A3 indicates the multiple bridge which handles one channel only, and 

 shows how the subranges are put in the circuit to handle the full range from 28° 

 to 90 °F. The eleventh contact is open to put that channel out of use. 



In the instrument, there are fifteen more bridges assembled similarly, one for 

 each channel, there being an automatic switching device in the recorder for in- 

 serting channels 1 to 16 into the potentiometer circuit consecutively. 



The third conductor in the cable is essential to eliminate error due to changing 

 resistance in the leads caused by change in temperature of the surrounding water. 



The switch ^i is a rotary, 11 -contact type, by means of which the desired tem- 

 perature subrange may be chosen from A to K. As the recorder prints, the specific 

 bridge having been brought into balance with the potentiometer, 5i is moved to 

 contact the next bridge circuit until it reaches K, and then a new cycle begins at A. 



The values for Ris and R^t must be calculated for each bead assembly, as their 

 function is to compensate for the differences in value of the resistances of the in- 

 dividual beads. 



The rotary switches (5i) have copper contact points with a multileaf spring 

 contactor mounted on a ceramic base. Ten of these contacts are connected, one to 

 each of the range resistors; the eleventh opens the arm of the bridge. These con- 

 tacts must be kept clean, as any extraneous resistance will introduce error in the 

 recorder reading. 



Associated with the bridge assembly is a source of potential, consisting of two 

 standard dry cells, a voltmeter, and potentiometer. These enable the operator to 

 maintain a constant potential across the bridge at all times, which is essential to 

 the accuracy of its operation. 



Recording Potentiometer 



All the above circuitry is contained in the upper part of a cabinet, the lower 

 part of which houses the recording potentiometer (fig. A4). 



The recorder employed is a Brown Recording Potentiometer No. 153-62, having 

 a full-scale travel time of 41/2 seconds. Its sensitivity is V4 per cent of scale span, 

 at 0.019 millivolts for spans less than 8 millivolts. The record is printed on a 

 strip chart of 11 inches recording width, having 150 divisions. The chart is driven 

 past the printing head by an electric motor, at a speed determined by the gear 

 ratio of the drive which may be altered to give any speed from 2 inches/hour to 

 480 inches/hour. Usually, a speed of 22.5 inches/hour was satisfactory (fig. A5). 



In this system, a slide wire potentiometer is driven by a screw mechanism to 

 balance the applied potential against a self-contained potential. When balance is 

 achieved, the printing wheel automatically prints a cross and a number, the num- 

 ber designating the channel being scanned at the moment. The circuit then auto- 

 matically switches to the next channel, and the process repeats with the next 

 consecutive number coming up on the printing wheel. The complete scanning of 



31 



