The recorder penmotor is designed to provide a linear pen movement 

 with a linear change in input current . 



The nominal current required to move the penmotor pen through a 50- 

 millimeter arc (full scale) is 20 milliamperes. The penmotor has a 

 nominal internal resistance of 1,500 ohms, thus by ohms law the constant 

 voltage source will be a nominal 30 volts (E = IR; .02 x 1,500 = 30 volts) 

 for the required pen movement. 



To make the penmotor move in equal increments from a variable resis- 

 tor made of several resistors connected in series as shown in Figure 6, 

 it is necessary to calculate each individual resistor value. 



CONSTANT VOLTAGE 

 (DC. Source) 





RECORDER 

 PEN MOTOR 





















K K 1 K 2 R 3 











• • • () 



Movahip Contnrt 





Figure 6. Series-Type, Step-Resistance Gage for Fresh Water. 



Assuming a wave gage with 20 equal measurement increments and a pen- 

 motor with an internal resistance of 1,500 ohms that requires 20 milli- 

 amperes for full-scale movement and that indicates linearly with a change 

 in current, then to move the pen full scale, 20 current changes of 1 

 milliampere each would be required. A 30-volt power source would provide 

 full-scale penmotor movement. 



30 volts 

 1,500 ohms 



,02 amperes 



The first of the 20 intervals of gage measurement would reduce the 

 20-milliampere penmotor current by 1 milliampere. 



To calculate the first gage resistor: 



30 volts 



19 milliamperes 



1,579 ohms. 



1,579 ohms total circuit resistance minus 1,500 ohm penmoter resistance 

 79 ohms (first resistor) . 



10 



