CHAP, iv.] ELECTRICITY AND MAGNETISM. 231 



resistance and electrostatic capacity of the cable have to 

 be met by balancing against them an " artificial cable " 

 consisting of a wire of equal resistance, and a condenser 

 of equal capacity. Messrs. Muirhead constructed for 

 duplexing the Atlantic Cable a condenser containing 

 100,000 square feet (over two acres of surface) of tinfoil. 

 Such condensers are also occasionally used on telegraph 

 lines in single working to avoid earth currents. They 

 are constructed by placing sheets of tinfoil between 

 sheets of mica or of paraffined paper, alternate sheets of 

 foil being connected together. Small condensers of 

 similar construction are used in connection with induc- 

 tion coils (Fig. 148). 



276. Practical Unit of Capacity. Electricians adopt a unit 

 of capacity, termed one farad, based on the system of electro- 

 magnetic units. A condenser of one farad capacity would be 

 raised to a potential of one volt by a charge of one weber of 

 electricity. l In- practice such a con- 

 denser would be too enormous to be 

 constructed. As a practical unit 

 of capacity is therefore chosen the 

 microfarad, or one millionth of a 

 farad ; a capacity about equal to 

 that of three miles of an Atlantic 

 cable. Microfarad condensers are 

 made containing about 3600 square 

 inches of tinfoil. Their general form 

 is shown in Fig. 106, which re- Fi g> I0 g t 



presents a \ microfarad condenser. 



The two brass pieces upon the ebonite top are connected re- 

 spectively with the two series of alternate sheets of tinfoil. The 

 plug between them serves to keep the condenser discharged 

 when not in use. 



Methods of measuring the capacity of a condenser 

 are given in Art. 348. 



277. Formulae for Capacities of Conductors 

 and Condensers. The following formulae give the 



1 See Note on Electromagnetic Units, Art. 321. 



