50 ANALOGIES 



Electrical resistance rs, in abohms, is defined by Ohm's law 



e 

 Ve = - 4.1 



where e = voltage across the resistance, in abvolts, 



/ = current through the resistance, in abamperes. 



B. Mechanical Resistance. — In a mechanical system dissipation is due 

 to friction. Mechanical resistance r^, in mechanical ohms, is defined as 



Vm = — 4.2 



u 



where /m = applied mechanical force, in dynes, and 



u = velocity at the point of application of the force, in centimeters 

 per second. 



C. Acoustical Resistance. — In an acoustical system dissipation may be 

 due to fluid resistance or radiation resistance. Fluid resistance is due to 

 viscosity. See Sec. 5.2. In the case of fluid flowing through a pipe with 

 a velocity of one cubic centimeter per second, resistance is represented by 

 the pressure drop along the pipe. The case of radiation resistance is dis- 

 cussed in Sec. 5.7. 



Acoustical resistance r^, in acoustical ohms, is defined as, 



r.^f- 4.3 



where p = pressure, in dynes per square centimeter, and 



U = volume current, in cubic centimeters per second. 



Volume current (sometimes termed volume velocity) is the rate of change 

 of volume displacement with time. In other words, volume current is the 

 linear velocity over an area multiplied by the area. 



4.3. Inductance, Inertia, Inertance. — A. Inductance. — Inductance is 

 the electrical circuit element which opposes a change in current. Induc- 

 tance is defined as 



di 

 e = L- 4.4 



dt 



where Z-= inductance, in abhenries, 



e = electromotive or driving force, in abvolts, and 



di 



— = rate of change of current, in abamperes per second 



dt 



