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The speed variations in a magnetic tape recorder resulting from the compliance of the tape and the inertias of the rotating parts were studied by means of the electrical analog of the mechanical system. The analog was also used as an "analog computer" to help evaluate certain mechanical parameters. Equations were derived for converting mechanical values into analogous electrical values, and vice versa. Mechanical values, including the tape compliance and the dynamic properties of the synchronous capstan motor, were measured, as well as the disturbances caused by existing mechanical imperfections. The "analog computer" method was used to determine tape-to-head responsiveness. Some "analog computer" values were compared with calculated values to show the accuracy and convenience of the "computer" method. Also, more realistic answers result from the "computer" method, because some simplifications necessary for a mathematical calculation are unnecessary for the "computer" solution. Speed variations at the reproducing head, and means for their reduction, were determined by using the analog circuit. Finally, a few general design rules were formulated.