Standard Resistance-coils with Mercury Standards. 345 



3. Method of Using the Standard,— -The standard is intro- 

 duced into the circuit by means of contact-pieces dipping into 

 the mercury in the cups. These pieces should not reach more 

 than a few millimetres below the surface. Contact-pieces of 

 amalgamated copper cannot be used, because they render the 

 mercury impure and alter its resistance rapidly, diminishing 

 it appreciably in a few hours. Platinum, again, does not 

 make a constant contact with the mercury; and the uncer- 

 tainty produced by its variability is fatal to its use. To avoid 

 this double difficulty, M. Benoit employed contact-pieces of a 

 special form (fig. 2). A glass tube A is Fig. 2. 

 drawn out to a point at its lower end ; 

 through this point a platinum wire B 

 passes. The wire is held in position by 

 shellac C ; and a small cup D of thin 

 glass is attached to its lower end also by 

 shellac. Thus the two separate glass 

 portions A and D are in electrical com- 

 munication by means of the platinum 

 wire B. The two portions A and D 

 are filled with mercury in a vacuum. 

 The mercury is thus brought into good 

 contact with the platinum, and the 

 mercury-platinum contact remains un- 

 changed during the observations. Into 

 the upper division A one end of a stout 

 copper rod E is plunged ; the other end 

 of this rod forms the connection with 

 the rest of the circuit. The cup D dips 

 a few millimetres below the mercury in 

 the standard. The mercury which fills 

 it remains constantly in contact with the 

 platinum wire, and may be kept without 

 change almost indefinitely. These contact-pieces introduce 

 an appreciable resistance of 2 to 3 thousandths of an ohm into 

 the circuit ; but this resistance remains constant, and can be 

 eliminated by a method of substitution. 



The standard is supported in a glass vessel, and can be 

 reduced in temperature to zero by filling the vessel with finely- 

 broken ice. The values of the standards are given at zero. 

 The value at any other temperature is given by the formula 



B t r= E (l + -0008649* + '00000112* 2 ), 

 t being the temperature in degrees Centigrade. 



4. Preparation of the Mercury. — The mercury may be 

 purified by the action of nitric acid, and dried by means of 

 sulphuric acid and caustic potash. 



