Two numbers of identical motors are mounted end to end. A flexible coupling joins their shafts. Both motors are provided with two numbers of 4mm sockets. Any motor may be used to drive the other as a dynamo. The whole unit is used for getting the degree of efficiency achieved when conversion of electrical energy to mechanical energy by a motor takes place. And also directly back to electrical energy by means of the motor to drive a dynamo. It is supplied complete with driving belt.