148 ANNUAL OF SCIENTIFIC DISCOVERY. 



local action into consideration, supposing each equally free from local 

 waste, is as follows,^ and the estimate is made up from actual experiment, 

 by computing the destruction of battery material in each necessary to 

 accomplish a given equal amount of work, say the deposition of a pound 

 of silver in the decomposition trough, Grove's, 32.^ cents ; Darnell's, 61 ; 

 Chester's, 21. 



NEW APPLICATION OF IBIDIUM. 



Mr. William McRea, of Philadelphia, has recently made a valuable 

 application of iridiiim, which consists in the construction of electro- 

 receiving magnets, with their contact surfaces of iridium. The metal 

 heretofore used for that purpose is platinum, which, although competent 

 to resist the action of the atmosphere, fuses quickly when exposed to the 

 electric spark from a powerful battery in a short circuit. As soon as the 

 platinum surfaces become in a slight degree oxidized, the points adhere to 

 each other, even when the main circuit is broken. This we may set down 

 as one of the causes of mistakes which sometimes occur in the transmis- 

 sion of a message by the Morse Telegraph, the alphabet of which consists 

 of dots, spaces and lines. It is evident, therefore, that if the points of 

 contact of the receiving-magnet, where the main circuit of the line is 

 broken by the transmitting operator for the purpose of making a space, 

 be impaired, that a dot and line, or dots alone, may run together, thereby 

 forming upon the paper a letter or character quite different from the one 

 intended by the transmitting operator. 



Iridium being the most indestructible of metals, the advantages of its 

 application in this instance are, that the surfaces of contact will last for 

 a much longer time without requiring any change, and will secure a more 

 perfect and unvarying surface of contact for the passage of the curi ent, 

 and prevent the adhesion of the points of contact together. Journal 

 Franklin Institute, July, 1851. 



IBIPKOVEMENT IN TELEGRAPHIC INSULATION. 



An improvement of value has recently been made by Mr. Dealing, of 

 England, in respect to the insulation of wires intended for telegraphic 

 communication. Heretofore, in constructing electric telegraphs where the 

 whole circuit has been made of metal, and also where the conducting 

 property of the earth has been employed as a part of the circuit, it has 

 been considered necessary to cause the wires to be thoroughly insulated 

 each from the others and from the earth ; the consequence of which has 

 been, that the expense of laying down electric circuits for electric tele- 

 graphs has been very great, particularly where the same have crossed the 

 sea, or other waters, where not only have the wires been insulated one 

 from another, and from the water or earth, but, in order to protect the 

 insulating matter from injury, great expense lias been caused by the use 



