MEASUREMENT OF HEIGHTS BY RESISTANCE ELEMENTS 



The following article was first published in 

 limited issue as Technical Report HE-II6-27O5 

 Fluid Mechanics Laboratory-j University of Calif- 

 ornia o It is reproduced here to acquaint re- 

 search workers and others who would not other- 

 wise know of the work with the valuable results 

 of the studyo The paper was prepared by MTo Jo 

 Ro Morison, University of Calif or nia o 



Introduction 



In measuring wave heights for model studies, it has been found 

 that wire elements or electrodes placed in the water act as a varia- 

 ble resistance for a recording systemo Through many experiments, a 

 definite method has been established for this 1^e o£-wave height re- 

 cording o It is the purpose of this paper to summarize the method of 

 measuring wave heights with wire elements recorded by an oscillograph » 

 Further, some of the difficulties and limitations of the method will 

 be presented for the experimentero The essential difficulty of the 

 method is in obtaining a stable, correct, and characteristic calibra- 

 tion of the system usedo 



General Measurement of Wave Height 



The measurement of one wave height at just one point by electrodes 

 may be accomplished in several wayso A two wire element fed by a 

 transformer and powerstat and connected in series to the galvanometer 

 of an oscillograph is a very satisfactory method o The calibration 

 curve in this case is practically linear depending, of course, on the 

 actual experimental conditions and instruments usedo It is also 

 possible to use the side of a metal tank as one of the wires of the 

 electrode o The calibration curve in this instance is not linear » 



The immediate questions arising from the above statement ares 



(1) What is the effect of the distance between the electrodes?; and 



(2) What type and size of electrodes are being discussed? The 

 distance between the electrodes when small (less than one-half inch) 

 is a factor in determining the resistance that is in the circuit,, 

 "When the electrodes are farther apart, the resistance is determined 

 by the water to wire relationship o A small diameter (Oo033 inches) 

 wire in water gives a higher resistance than does a larger diameter 

 wire (1/16 inches) o It has been found that a distance of approximately 

 one-half inch (range J to 1) is very satisfactory for two wire 

 elements and distances up to 6 inches have been used for the case 

 where the metal tank is one electrode o Further consideration of the 

 resistance of the water between the wires and how the circuit actually 

 operates indicates idiat the wire and water act together in taking 

 resistance out of the circuito This resistance starts near infinite 

 (electrodes in air) and is immediately reduced to several thousand 

 ohms when the wires touch water (the magnitude of this resistance 

 depends on the distance between the electrodes, the solution, and the 



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