THE VOLTAIC ARC. 



21 



up in that country by a Commission of distinguished electri- 

 cians, such as Tyndall, Douglass, Sabine, &c., it is stated that 

 the concavity formed in the positive electrode can, by a 

 proper disposition of the negative carbon, be made to furnish 

 what is termed a condensed light, which considerably increases 

 the light emitted in a given direction. Consequently, accord- 

 ing to these gentlemen, the light supplied by rectified cur- 

 rents is much preferable 

 to that which results 

 from currents alternately 

 reversed. We give far- 

 ther on, however, in a 

 table drawn up by Doug- 

 lass, the difference of 

 intensity of the two 

 lights, with or without 

 rectification of the cur- 

 rents. In order to ob- 

 tain the best possible 

 effects it is necessary, 

 according to Douglass, 

 that the lower carbon, 

 which is the negative 

 one, should be placed as 

 shown in Fig, 3, so that 

 its axis may be in the 

 prolongation of the side 

 of the upper carbon, which faces the part of the horizon re- 

 quired to be illuminated; the concavity, the bottom of which 

 is the most luminous part of the arc, then acts like a re- 

 flector, and its brilliancy is not concealed by the edges of 

 the crater round the luminous centre. " On account of 

 their loss of current and feebler action," says Douglass, " the 

 machines of the Alliance Company and those of Holmes are 

 far from being equivalent to the new machines with rectified 

 currents, and in the application of the electric light to light- 



FIG. 3. 



