166 ANNUAL OF SCIENTIFIC DISCC/VERY. 



used should be darkened. The instrument is appropriate to ther- 

 mo-electric currents and those of exceedingly feeble intensity. 



TELEGRAPH INSULATION. 



Mr. Varley, well known for his skill as an electrician, and 

 especially for his services in connection with the Atlantic cable, 

 has lately obtained the following patent : — 



Insulators for telegraphic wires are usually made with an iron 

 pin, coated with what is known as vulcanite, or hard vulcanized 

 India-rubber, and secured by means of plaster of Paris, or other 

 cement, inside of a porcelain, or other earthenware cup inverted. 

 As heretofore made it has been found that the vulcanite covering 

 is liable to be porous, and full of what are known as blow-holes, 

 and that the iron or steel pin is liable to rust, by reason of the 

 presence of the vulcanite covering, and that the accumulation of 

 the rust is liable to crack the vulcanite covering. 



And the first part of said invention relates ta a method of pre- 

 venting the pin from rusting, and consists in coating the iron or 

 steel pin with zinc, and then coating the zinc with tin, or an 

 alloy of tin, preparatory to applying the vulcanite covering. The 

 zinc is applied in the well-known manner of galvanizing iron, and 

 the zinc is coated with tin, or an alloy of tin, by dipping in a bath 

 of molten tin, or alloy of tin, in the same manner as sheets of 

 iron are usually tinned ; and, after being so tinned, the preparation 

 of India-rubber, or other vulcanizable gum, mixed with sulphur, 

 is applied in the green or plastic state, and then subjected to the 

 vulcjinizing heat to be hardened, in a manner well known to manu- 

 facturers of vulcanite. When so made, the vulcanite will be 

 found to be solid and without blow-holes, and the pin will not rust 

 however much it may be exposed. The cement used for securing 

 the pin inside the inverted cup is more or less porous, and the 

 presence of such pores renders the insulation imperfect. 



And the second part of said invention, which relates to a method 

 of avoidinof such defect, consists in saturating the cement with 

 paraffine wax, to fill up the pores. In practising the second part 

 of said invention, the iron pin, with its covering of vulcanite, is 

 inserted in the cup, without touching any part of the surface 

 thereof except the bottom of the inside of the inverted cup, and 

 the intervening space is filled with cement, made, by preference, 

 of 1 part of plaster of Paris and 2 parts of Portland cement; but 

 other cement may be used, such as plaster, or Roman or Portland 

 cement alone. After the parts have been united by the cement, 

 the whole is put in a bath of melted paraffine wax, at a tempera- 

 ture of about 224° F., and there left until all bubbling ceases, 

 which indicates that all the water has been expelled from the 

 cement, and that the pores have all been filled. In this way all 

 the pores and interstices are filled with paraffine wax, thereby ren- 

 dering the insulation more perfect, and as there are no pores into 

 which moisture can lodge, there will be no danger of fracturing 

 the parts by the expansion of water in freezing, as heretofore. 



