An electrical device is described for evaluating the series &Sgr;Aisinaix.For each term in the series there is constructed one compounding element composed of three coils; a long solenoid acting as a primary (60‐cycle a.c.) within which are mounted two short secondary coils, one a rotor (order of 1/5 r.p.m.) and the other a stator. The mutual inductance between the primary and rotor varies as the sine of the angleaixof rotation while the primary current is held proportional toAi.The vector sum of the peak voltages induced in the rotor and stator varies about a constant value proportional toAisinaix.Thus the variations of the resultant voltage from all of the elements about a constant value are proportional to the sum of the series. A record of the sum is made on photographic paper by means of a vacuum tube voltmeter circuit. Satisfactory tests have been made on a machine containing two elements. The application to structure determinations of gas molecules by electron diffraction is presented.