1879.] 



On the Capillary Electroscope. 



35 



it may be drawn out to a sufficient degree of fineness at one heating ; 

 that having a circular bore it is less likely to become clogged by use 

 than that which is oval ; and it is an advantage to employ that which 

 has white enamel at the back. Success in making the capillary 

 depends largely upon careful manipulation and the employment of a 

 flame of suitable size ; I have used a Bunsen flame about an inch in 

 height and half an inch in diameter, for drawing long capillaries by a 

 single heating, or a very minute flame of a small spirit-lamp for draw- 

 ing out a short one by a second heating. Within certain limits, I 

 have found that the finer the capillary tube the greater is the move- 

 ment in it of the meniscus by a given electromotive force, and the 

 greater the pressure required to balance that force ; but if the bore of 

 the tube be extremely fine, say *01 millim., the instrument becomes 

 less sensitive, probably owing to increased adhesion or to increased 

 conduction-resistance of the minute column of solution. A convenient 

 bore is about '03 to "04 millim., and an external diameter of "3 to '4 

 millim. Under uniform pressure, the meniscus moves not only 

 through a much greater length, but also at a much faster rate, in the 

 finer portion of a tube of moderate diameter, than it does in the 

 coarser part, and this occurs even in tubes the internal diameter of 

 which diminishes to their very extremity. Tubes of parallel bore are 

 not usually suitable for the instrument (especially if the latter is used 

 in a vertical position), because, instead of presenting a resistance 

 gradually increasing downwards, as they should do, they offer the 

 reverse. The best form of capillary is one which becomes narrower 

 at a sufficiently increasing degree towards its free end. I have em- 

 ployed capillaries varying in length from 5 to 15 centims. with equal 

 advantage. 



Considerable pressure is often requisite for filling the capillary tube, 

 and for overcoming obstructions ; to produce it, a compressible pres- 

 sure-chamber is much more convenient than a tall column of mercury. 

 An excellent chamber may be formed of a piece of vulcanized india- 

 rubber tubing, about 6 or 8 centims. long, 3 millims. bore, and 15 millims. 

 external diameter, such as is employed as " engine-packing." It is 

 more convenient to have the platinum wire (for connexion with the 

 column of mercury) inserted through the side of this tube than through 

 that of the glass one, because the former does not so readily cause the 

 wire to be broken. To insert the wire, one end of the tube is stretched 

 open by means of a small pair of pliers ; a needle, wet with glycerine, 

 is then forced very obliquely through the india-rubber close to the end 

 of the tube, and after its removal a fine wire may be readily forced 

 through the minute stretched hole and the pliers then withdrawn. 

 After the capillary tube has been fixed in this chamber (which must 

 be previously made scrupulously clean), the end of the chamber 

 adjacent to the terminal and capillary must be securely bound round 



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