Section II. SERIES-TYPE, STEP -RESISTANCE GAGE FOR USE IN FRESH WATER 



1, Theory of Operation of Series-Type, Step-Resistance Gage 



The series- type, step-resistance gage operates on the principle 

 of a simple series-type circuit with a constant voltage d.c. source, a 

 recorder penmotor and a variable resistor connected in series. Figure 5 

 shows a simple series circuit containing a constant voltage source, the 

 penmotor, and a variable resistance. 



CONSTANT VOLTAGE 





RECORDER 





Recorder Pen 



(DC. Source) 



PEN MOTOR 

























N^<^ 









Variable Resistor 



Figure 5. Series-Type, Step-Resistance Gage for Fresh Water 



The recorder penmotor is essentially the same as an ordinary 

 D'Arsonval panel-meter movement with a capillary, ink pen substituted 

 for the indicating pointer. The components (magnet, moving coil, springs, 

 etc.) are significantly larger and more rugged than the ordinary meter 

 movement . 



The variable resistor used in a gage consists of several fixed re- 

 sistors in series with contact tips connected to the resistor junctions. 

 The contact tips are molded into long epoxy resin shapes that allow the 

 contact tips to be exposed. The number of contacts submerged by the 

 part of the gage that is under water forms the variation in resistance. 



The constant voltage d.c. source is obtained from a transformer, 

 rectifier, filter, and voltage regulator operated from a 115-volt, 60- 

 cycle power line. The output voltage from these components is constant 

 even though the input line voltage may vary from about 95 to 130 volts 

 and thus prevents the recorder penmotor from changing with a varying 

 line voltage. 



Since the voltage source is constant and the resistance of the pen- 

 motor is constant (neglecting temperature changes) , the current in the 

 circuit will increase if the value of the variable resistor is decreased. 

 Ohms law, E = IR, applies. 



