272 SOME NEW MODES OF LIGHTING. 



ingenious manner b^^ means of a "^Mieatstone bridge, made up of the 

 two side arcs themselves and two suitabU^ resistances, which are 

 wound as electromagnets in such a manner as to operate the regulat- 

 ing mechanism. The central cylinder thus carries the current only 

 when the bridge is thrown tem])()rarily out of balance. 



Instead of rendering the arc luminous by the aid of refractory 

 oxides, the same result may be obtained by introducing a combustible 

 liquid or })owder through a hollow electrode. This device has been trit^d 

 by J. Akermann, but it does not seem likely to prove very j^racticable. 



All these devices of introducing foreign substances into the arc are 

 evidently for the doul>le purpose of diminishing its resistance and of 

 augmenting its luminosity, and there seems to be no reason why a 

 conducting vapor like that of mercury shoidd not be substituted for 

 the solid particles of carbon or of metallic salts or oxides which have 

 been used for this purpose. The Cooper-Hewitt lamp is the develop- 

 ment of this idea. Everybody is familiar with the Geissler tubes, in 

 which light is produced by an electric current traversing a rarefied 

 gas. It is possible, as Tesla has shown, to excite such tubes to lumi- 

 nosity by simply placing them in a varying electrostatic field of high 

 frequency. About 18i)t2 Arons showed that mercury vapor traversed 



Self 



Q 'OOOOOOOOOOOUOOOOOOODD" 



o 'mmmmmmmmm>- 



'K(*sislancc 



Fig. 3.— Disposition of current in Cooper-Hewitt lamp. 



by a current emits light, but there was no idea at that time of employ- 

 ing this device for illuminating purposes. The credit for this discov- 

 ery rests, therefore, with Cooper-Hewitt, who has devised a mercury 

 lam]) of great simplicity, composed of a glass tube with electrodes at 

 the two ends. The negative electrode is mercury itself, and the posi- 

 tive electrode may be either mercury, iron, or other metal. Platinum 

 wires sealed into the glass conduct the current to the electrodes. 

 When cold, the pressure of the mercury vapor is very small, but when 

 heated by the passage of the current the mercury va})or pressure rises 

 to about 2 mm. of liquid mercury, or about one four-hundredth of 

 an atmosphere. 



Cooper-Hewitt lamps operate under a difference of potential of 

 110 volts, of which about 80 are absorbed in the arc. Since the inter- 

 nal resistance is very high at first, this potential difference is insuf- 

 ficient to start the light, and sevei-al thousand volts are recpiired for a 

 brief interval at the start. Accordingly some auxiliary device has to 

 be provided, as is the case with the Nernst lamp, to heat the con- 

 ductor to the temperature at which the ordinary voltage will main- 

 tain incandescence. For example, the required voltage may be ob- 



