ELECTRICAL SCIENCE. 165 



are delivered from and returned to the machine alternately in opposite direc- 

 tions, and canse them to pass to and through a conductor in one and the same 

 direction as it' induced by a battery. The details of the invention are voluminous. 

 — Mechanics' Magazine. 



HEATING AND TESTING OF TELEGRAPH CABLES. 



Mr. C. W. Siemens, in a letter to Professor Tyndall, in the 

 Philosophical Magazine, describes how he has availed himself 

 of a striking and most fortunate application of a well-known 

 fact in electricity, that the resistance encountered by an electric 

 current in passing through a wire, is augmented in proportion to the 

 augmentation of temperature. Conversely, we can accurately infer 

 the increase of temperature from the increase of resistance, and this 

 is the principle which Mr. Siemens has so happily applied. He had 

 charge of the Rangoon and Singapore telegraph cable, and was led 

 by previous observation to surmise that a spontaneous generation of 

 heat sometimes took place when large lengths of such cables are 

 formed into coils. He was, therefore, anxious to keep himself 

 acquainted with the temperature of the inner portions of his coil, but 

 could not, of course, introduce ordinary thermometers there. He 

 introduced, however, between the layers of the cable at regular 

 intervals suitable coils of copper wire, the resistance of which for a 

 long series of temperatures had been determined beforehand. The 

 ends of these copper coils issued into the air, so that they could be 

 connected at any time with a suitable apparatus for determining 

 their resistance. Now, Mr. Siemens found that although the outer 

 portion of the coil of cable had a temperature not sensibly higher 

 than that of the air, the wires which he had placed within the coil 

 showed a steady augmentation of resistance, from which he inferred 

 that the cable was heating within. He waited until the augmented 

 resistance indicated an increase of temperature from sixty to eighty- 

 six degrees. Had he waited much longer, the cable would probably 

 have been destroyed. Some of those to whom he communicated his 

 conclusions regarded them for a time as the mere refinements of 

 theory, butall their doubts were dissipated when a quantity of water, 

 at a temperature of forty-two degrees, thrown upon the top of the 

 cable, after passing through the inner portions of the coil, issued 

 from the bottom with the temperature raised to seventy-two degrees. 



The precise cause of this generation of heat has not, we believe, 

 been yet determined. It may be due to some chemical action in 

 the gutta-percha ; but it may also be due to the gradual rusting 

 of the iron which encases the cable. "Who can say (asks a writer 

 in the Saturday Review) what injury was done to the gutta-percha 

 covering of the Atlantic cable, through ignorance of the fact so 

 opportunely observed in the case of that of Rangoon and Singapore f 



This cable has been tested by a process invented by Mr. W. Reid. 

 During the manipulation of gutta-percha, a great quantity of air and 

 water is liable to become mixed with it, and in marine cables, when 

 covered with ropeyarn and iron wire, these faults have hitherto been 

 detected only after submersion. By the plan of Mr. Reid, however, 



