The Elements of Electro-Technics (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1920 edition. Excerpt: ... the physical explanation. It is assumed that the motion is from left to right. The induced current is flowing towards the observer, whilst the resultant magnetic field is clearly producing a drag on the conductor from right to left. 26. A similar force is acting on any one of the conductors on the armature of a dynamo when the machine is developing current.. If we had a perfectly frictionless machine, and there were no other losses of any kind--that is, if the efficiency was 100 per cent.--then the driving torque supplied by the prime mover coupled to the dynamo would be determined simply by this electro-magnetic force F. It was explained in the previous chapter that this force is directly proportional to the current, and we therefore see how it is that, as the current delivered by the dynamo is increased, it is necessary for the prime mover to exert a greater driving torque, and therefore develop more power. Actually, owing to frictional and other losses, the power developed by the prime mover does not increase in strict proportion to the load on the dynamo (that is, the product of current and volts), but very nearly so. The Eddy-current Brake 27. It was observed by Arago, in 1824, that the oscillations of a compass needle were damped by placing it in a jar fitted with a copper base. He also rotated a copper disc beneath a compass needle and found that the latter was acted upon by a force which caused it to rotate in the same direction as the disc. Babbage and Herschel repeated Arago's first experiment, using different metals, and found that the damping force was greatest with silver and copper; that is, with the two metals which are the best conductors. They also reversed Arago's disc experiment, and produced rotation of a copper disc by...