- G is the goodness factor (factors above 1 are likely to be efficient)
- Am, Ae are the cross sections of the magnetic and electric circuit
- lm, le are the lengths of the magnetic and electric circuits
- μ is the permeability of the core
- ω is the angular frequency the motor is driven at
- σ is the conductivity of the conductor
From this he showed that the most efficient motors are likely to be relatively large. However, the equation only directly relates to non-permanent magnet motors.
Laithwaite showed that for a simple induction motor this gave:
- ER Laithwaite (1965). "The Goodness of a Machine". Electronics and Power. 11 (3): 101–103. doi:10.1049/ep.1965.0071.
- DJ Patterson; CW Brice; RA Dougal; D Kovuri (2003). "The "Goodness" of Small Contemporary Permanent Magnet Electric Machines" (PDF). Proceedings of the International Electric Machines and Drives Conference. 2: 1195–1200. doi:10.1109/IEMDC.2003.1210392.
- ER Laithwaite (1965). "Electromagnetic levitation". Electronics and Power. 11 (12): 408–410. doi:10.1049/ep.1965.0312.