CHAP. VIL] ELECTRICITY AND MAGNETISM. 339 



nearly in inverse proportion to the square of the distance 

 from its poles, the attracting force can only produce 

 effective motion through very small distances. 



In 1839 Jacobi propelled a boat along the river Neva 

 at the rate of three miles per hour with an electromagnetic 

 engine of about one horse-power, worked by a battery of 

 64 large Daniell's cells. 



The dynamo-electric machines of Gramme, Siemens, 

 and others, described in Arts. 407 to 411, will also work 

 as electromotors, and, indeed, are the most efficient of 

 electro-magnetic engines. 



376. Electric Transmission of Power to a 

 distance. The increasing use of dynamo -electric 

 machines for electric lighting has revived the problem of 

 transmitting power to a distance by electrical means, 

 and so utilising waste water-power. A mountain stream 

 may be made to turn a water-wheel or turbine, and 

 drive a dynamo - electric machine, thereby generating 

 currents which can be conveyed by wires to an electro- 

 motor at a distant point, and there reconverted into 

 mechanical power. Whether such transmission is profit- 

 able or not depends on the efficiency of the machines 

 employed. 



377. Jacobi's Theory of Electromotors. If a 

 galvanometer be included in a circuit with a battery and 

 an electromotor, it is found that the current is weaker 

 when the electromotor is working than when the electro- 

 motor is standing still, and that the faster the electromotor 

 runs the weaker does the battery current become. This 

 is due to electromagnetic induction (Art. 391) between the 

 moving and fixed parts of the electromotor, which, as it 

 spins round, generates a back-current. The electromotive- 

 force due to this inductive action increases with the speed 

 of the electromotor, so that the back-current is strongest 

 when it runs fastest. If the motor be loaded so as to 

 do work by moving slowly against considerable forces, the 

 back-current will be small, and only a small proportion 



