256 THE POPULAR SCIENCE MONTHLY. 



great fall in the insulation resistance, and a slight fall in the apparent 

 resistance of the copper conductor, between the two stations ; but 

 messages can be still transmitted, as a part only of the whole cur- 

 rent, inversely proportional to the resistance of the fault, escapes into 

 the ocean. If one office insulates the cable, and the other measures 

 the resistance, the fault acts like a fault that is caused by the fracture 

 of both the copper wire and the gutta-percha, but little of the copper 

 core being exposed. 



The fifth kind of fault corresponds almost exactly in behavior 

 to a fault caused by fracture of the copper conductor and gutta- 

 percha, in which a considerable portion of the length of copper wire 

 remains exposed to the water. The resistance will vary still less ; 

 and there will be a total absence of the feeble currents which result 

 when the copper and iron of a cable are broken and separated by salt 

 water. 



Submarine or ocean telegraphy holds a very prominent place in 

 electrical engineering, and the instruments used in it are interesting. 

 In instructing pupils a very curious apparatus is used. It is the arti- 

 ficial or dummy cable, consisting of a number of "resistance-coils," 

 and condensers so arranged as to reproduce all the phenomena and 

 all the practical difficulties that are presented by a real ocean-cable. 

 With a good instructor, this piece of apparatus is of very great ser- 

 vice, inasmuch as all kinds of imperfections can be readily and cor- 

 rectly imitated in any part of the circuit. 



Still greater interest, perhaps, attaches to the apparatus for show- 

 ing the retardation that a current experiences in traversing a long 

 cable. This apparatus consists of a series of "resistance-coils," "rheo- 

 stats," and condensers, having small receiving instruments at a dozen 

 different points in the circuit, representing as many different offices on 

 the line. The receiving instruments are similar to the mirror portion 

 of Sir William Thomson's mirror galvanometer. In this a ray of light 

 falls upon a very small mirror attached to a small magnet ; and this 

 rotates around a vertical axis when acted upon by a current that cir- 

 culates in a coil of wire. These magnets, with the mirrors attached^ 

 moving one after the other, indicate the time taken in charging the 

 whole length of the circuit. 



I. The Storage of Electricity. Another princi]:>al branch of 

 electrical engineering, promising much in the near future, is the great 

 French discovery of the storage of electrical energy. It is among the 

 most important inventions of the last thirty years. The electrical 

 storage of energy must not be confounded with the storage of elec- 

 tricity. An electrical storage-battery is an apparatus for transform- 

 ing electricity ; in it electrical energy is no longer produced directly, 

 but changes its properties. A given source furnishes a certain vol- 

 ume or quantity of electricity, at a certain pressure or tension. In 

 certain instances, it may be important to increase one of these prop- 



