206 APPLIED SCIENCE 



as the difference between the velocity of light and that of a 

 body moving through one foot in six thousand seven hundred 

 years ; yet the measurements of the two quantities are daily 

 made with the same apparatus, and the same standards of com- 

 parison. This fact is well calculated to give an idea of the 

 range of electrical measurements and the perfection to which 

 the instruments employed have been brought. 



Resistance coils and the galvanometer variously combined 

 allow these measurements to be accurately made in many ways. 

 Sir W. Thomson's reflecting galvanometer is now almost exclu- 

 sively used for this purpose. The simple deflection test is still 

 frequently employed, but it is then reduced by calculation so as 

 to give the results in resistance. 



It would be out of place to attempt to explain in detail the 

 modes of testing adopted, but it may be interesting to enumerate 

 the several examinations which each mile of insulated wire 

 undergoes before it is admitted to a cable. 



1 . The hank of copper wire is tested for resistance. 



2. The resistance of the copper conductor of the insulated 

 mile of wire is measured after having been kept for twenty-four 

 hours in water at a constant temperature. 



3. The resistance of the insulator is measured under the 

 same conditions, once with a current from the zinc pole, and 

 once with a current from the copper pole of the voltaic battery. 

 The above tests are made by the contractor. 



4. 5. The last two tests are repeated by independent observers 

 acting as the engineers of the company. 



6. The coil of wire is again tested for insulation immediately 

 before being joined to the manufactured cable. 



In addition to these tests, in many cases the insulation is 

 tested in water under a great pressure, to simulate the pressure 

 occurring at the bottom of the sea. This test was patented by 

 Mr. Reid, and is probably of considerable service, although in 

 the vast majority of cases the insulation resistance is increased 

 by pressure. While a cable is being submerged, it is indeed 

 customary to expect an improvement of about 7 per cent, 

 for every 100 fathoms of water, due to this cause only; thus in 

 2,000 fathoms an improvement of 140 per cent, is expected. 



After the cable is sheathed with iron, it lies under water in 



