212 Mr. J. J. Manley on the 



their seats with emery-dust paper ; and as a further aid to 

 constancy each plug was used only for that seat in which it 

 was found when the coils arrived from the makers. 



When a plug is seated by the usual screw-like movement, 

 the engaging surfaces tend to plough each other, and as a 

 result burrs and grooves appear upon both. When a bun- 

 has been formed, its presence is easily detected by the charac- 

 teristic gritty feeling which accompanies any displacement 

 of the plug : the existence of a groove is readily verified with 

 the aid of a lens. These burrs and grooves, which change in 

 size, number, and form as the coils are used, appear to be 

 entirely responsible for the variations observable in plug- 

 resistance. 



Now a too frequent re-polishing of the plugs, besides being 

 highly inconvenient, is prejudicial to accuracy : we were 

 therefore led to try the effect of several lubricants, and the 

 plan was immediately successful. 



In reply to an inquiry, Dr. Glazebrook, however, informs 

 me that this method is not new, and that Mr. Price some 

 12 or 14 years ago showed that contact resistance is reduced 

 by the application of petroleum either in the form of a film 

 or bath, and that for some years past it has been the custom 

 at the National Physical Laboratory to lubricate box-plugs 

 and switches with paraffin ; and although no special de- 

 scription of the method has been published, allusions to the 

 device have been made in various papers dealing with 

 matters of a kindred nature. Under these circumstances it 

 is believed that an account of some quantitative experi- 

 ments recently carried oat with the aid of five boxes of coils, 

 may be helpful to other observers. 



To begin with, the plugs and seats of box No. I. were 

 polished and wiped with a clean cloth. Next, strips of stout 

 writing-paper were, with the aid of resin cerate, fixed to the 

 vertical faces of the row of brnss blocks. As the upper 

 edges of the paper were somewhat higher than the top of the 

 blocks, a trough was thus formed, and this was filled with oil 

 of the kind used in a Geryk pump. All the plugs having 

 been normally seated, their joint resistance was measured by 

 Carey Foster's method ; the plugs were then loosened, 

 re-seated, and their resistance again determined : these 

 several operations were performed 10 times, and from the 

 10 independent values a final mean value was calculated. 



Secondly, the strips of paper were removed and the oil 

 allowed to drain away, but the plugs and their seats were not 

 wiped : a new series of measurements was then effected, and 



