158 METALLIC COATING FOR ELECTRIC RUBBERS. 



amalgam, 150 revolutions were sufficient. This amalgam is 

 used by triturating it in a mortar and mixing it with lard. 



The annexed extract from the report of my friend, Mr. Jas. 

 Green, to whom was referred the determination of the relative 

 values of these two substances as promoters of electrical 

 excitement, will enable experimenters to judge for themselves 

 of the comparative advantages of each. After detailing the 

 mode of experimenting, Mr. Green's report proceeds : 



'In that condition of the atmosphere most favourable for 

 electrical excitation, I could discover no difference in the 

 amount of the effect produced by the two substances : but in 

 a moist and unfavorable condition of the air, I found the effect 

 greatly in favour of the amalgam. This difference augment- 

 ing with the difficulty of producing electrical development. 



'The advantages attending the use of the bisulphuret of tin 

 over the ordinary amalgam, are, however, so great, that I 

 think it in common application, the most acceptable material. 

 Every one accustomed to use the electrical machine must have 

 felt the trouble attending the use of amalgam, particularly if 

 the machine has been idle for any length of time : the surface 

 of the rubber becomes hard and requires more or less labour 

 to put it in fit state for use, the cylinder or plate becomes 

 occasionally streaked and greasy, which holds the dust tena- 

 ciously, and then the trituration of the amalgam upon every 

 application is somewhat troublesome. Now, these inconve- 

 niences do not attach to the use of mosaic gold, it adheres to 

 the rubber without lard, does not streak the glass and is 

 readily renewed on the rubber, by rubbing a lump over its 

 surface, or by spreading it over with powder. These advan- 

 tages, with its superior neatness, render it admirably adapted 

 for the larger class of electrical machines.' 



