Sec. 1-3] MECHANICAL INPUT TRANSDUCERS 93 



uses a rotating yoke of iron or other material with high magnetic 

 permeability, which alternatingly increases and decreases the flux 

 passing through the coil. An alternate form in which an iron body 

 rotates in the gap between a stationary magnet and a stationary coil 

 is shown by Pflier. 1 



The output voltage from all three systems is proportional to the 

 rotation velocity, provided the load resistance connected to the ter- 

 minals is very high. If the load current is not negligible, a magnetic 

 field arises in the coil which can distort the primary (exciting) mag- 

 netic field and give rise to nonlinearity. Errors may arise from tem- 

 perature variation. The residual flux and the coercive force of 

 permanent magnets decrease in general at higher temperatures. The 

 effect can be reduced by using electromagnets instead of permanent 

 magnets and compound (shunt-type) windings. Measurement of the 

 output freqtiency from a-c generators can furnish more accurate 

 results; however, since most frequency-measuring systems de- 

 mand constant voltage, means have to be provided to keep the 

 output voltage from the tachometer constant, i.e., independent of 

 the rotation velocity. This can be accomplished either with com- 

 pound windings similar to those used in automobile generators or by 

 a subsequent stage, e.g., a driven multivibrator. The most accurate 

 results can be obtained by the use of digital systems, i.e., by counting 

 the number of cycles during a measured time interval. 



c. Separate Measurement of As and At. If a body moves through a 

 distance As during a time interval At, a velocity-proportional signal 

 can be obtained by producing electric signals proportional to As and 

 At and forming the ratio As/ At = v by electrical means. The method 

 is usually modified by keeping As constant and measuring At elec- 

 trically, or vice versa. An example of a circuit used to determine the 

 velocity of a bullet or an explosion wave is illustrated in Fig. (1-3)6. 

 The two switches (or thin wires) A and B are a distance As apart and 

 are closed initially. Opening of the switch A causes the appearance 

 of a voltage between the output terminals (the voltage drop across 

 E 2 ) which disappears at the opening of the switch B. The length of 

 the voltage pulse is A? and is inversely proportional to the velocity. 



Numerous variations of this method have been described, in partic- 

 ular in connection with repetitively acting devices such as in tachom- 

 eters. Either one or several switches or commutators are used; 

 either the pulse integral or the average pulse height is measured (e.g., 

 in "point dwell meters" for the examination of ignition systems in 



1 P. M. Pflier, "Elektrische Messung mechanischer Grossen," 3d ed., p. 40, 

 Springer Verlag, Berlin, 1948. 



