272 



SOME NEW M0DP:S OK r^TCllTlNG. 



iiiox.iiious iiiiiniicr by incaiis of a Wheatslone bridge, mado up of the 

 (wo side ai'cs UuMusolves and two suitable resistances, which are 

 wound as ek'ctroinaonels in such a manner as to operate tlie i-e<ruhit- 

 inu" ni(M hanisiu. The central cylinder thus carries the cuiTent only 

 when Ihe l)rid<ie is thrown temporarily out of balance. 



Instead of rendering- the arc luminous by the aid of icfractory 

 oxides, the same i-esull may be obtained by introducing a combustible 

 li(]ui(l or powder through a hollow electrode. This device has been tried 

 by ,1. Akermann, but it does not seem likely (o prove very practicable. 



All these devices of iuti'oducing foreign substances into the arc are 

 evidently for the double purpose of diminishing its resistance aiul of 

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

 conducting vapoi' lik'e that of mci-cury should not be substituted for 

 (he solid particles of carbon or of metallic salts or oxides which have 

 been used for this purpose. The (\)oi)er-riewitt lamp is the develop- 

 ment of this idea. Kveiybody is familiar with the (leissler tubes, in 

 which light is produced by an electric current traversing a rarefied 

 i>as. It is ])ossibl(>, as Tesla has shown, to excite such tubes to lumi- 

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

 frequency. About 1(S92 Arons show^'d (hat mercury vapor traversed 



KrMStancc 



Fid. :i. I)isi)iisiti()ii of <'nrrcnt in (\)()i)er-H('Witt liiiiii). 



b\' a cui-reut emits light, but there was no idea at (lia( (ime of employ- 

 iwj: (his (le\'ice for illiuuinadng purposes. The credit foi' this discov- 

 ei-v rests, thend'orc. with Co()perTIewi((, who has de\ ised a mercury 

 lamp of ureat simplicity, composed of a glass (ube with electrodes at 

 (he two ends. The negative elecd-ode is mercury itself, and the i)()si- 

 ti\-e electrode may be either mercury, iron, or other metal. Phdimun 

 wires sealed in(o the glass conduct (he curi'en( (o (he electrodes. 

 When cold, (he pressure of the mercury vapor is vei-y small, l)ii( when 

 hea(ed bv the passage of (he current (h(> mercury vapor pi-essure rises 

 to about -2 mm. of li(|uid mei-cui-y, or about one fonr-huudi-edth of 

 an atuu)sphere. 



Cooper-lb'wi(t lamps operate under a dilh'rence of [)oten(ial of 

 110 vohs, of which aboiU SO are absorbed in (he arc. Since (he iu(er- 

 nal resistance is very high at first, this ])otential diil'erence is insuf- 

 ficient to start the light, and several thousaiul volts are recjuired for a 

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

 be providecl, as is the case with the Nernst lamp, to heat (he con- 

 ductor to the tem])erature at which the ordinary voltage will nuiin- 

 tain incandescence. For e\am[)le, the re(|uire(l voltage nuiy be ob- 



