WIRELESS TELEPHONY FESSENDEN. 169 



manent magnet and heated by radiation from a wire, which in turn 

 was heated by the current to be detected) described by the writer 

 at the Columbus meeting of the American association in 1897.'* 

 This was abandoned in favor of Prof. Elihu Thomson's alternating- 

 current galvanometer,'' suitably modified for telegraphic work.*^ 



Among other forms of current-operated receiver may be mentioned 

 the following: 



The hot-wire barretter,** consisting of a minute platinum wire 

 a few hundred thousandths of an inch in diameter and approxi- 

 mately a hundredth of an inch in length. The term " barretter " was 

 coined for this device for the reason that it differs essentially from 

 the bolometer of Langley in that it is arranged to be affected by 

 external sources of radiant heat as little as possible instead of as 

 much as possible, and to have an extremely small specific heat, an 

 object not sought in the case of the bolometer. 



The liquid barretter,'^ in which the change of resistance is effected 

 by heating a liquid, the concentration of path being obtained by 

 means of a fine platinum wire point. Some question has been raised 

 as to the theory of operation of this device, but I think there is no 

 question but that the effect is due to heat, though what per cent 

 of the effect is due to change in ohmic conductivity by heat and what 

 per cent is due to depolarization by heat is still, as originally stated 

 by the writer,^ uncertain. The facts that the device operates prac- 

 tically equally well irrespective of which terminal is connected to 

 the local battery, and that the effect varies as the square of the 

 alternating current (as a heat-operated device should do) instead of 

 directly with the alternating current as a rectifier would do, and 

 that depolarization is produced by the heat, have been confirmed 

 by Dr. L. W. Austin.^ The writer has experimentally determined 

 the fact that though the electrical impulses may have a duration 

 of less than a millionth part of a second, the change in resistance per- 

 sists for approximately the ten thousandth part of a second, Avhich 

 would seem to show conclusively that the action is not a direct effect 

 of the waves. 



The term electrolytic receiver has sometimes been applied to the 

 liquid barretter. This is objectionable, as there are a number of 

 electrolytic receivers. For example, the Neughschwender-Schaefer ^ 



° Electrician, June 24, 1904. 



6 Elihu Thomson, United States patent No. 363185, January 26, 1887. 

 " United States patents Nos. 706736 and 706737, December 15, 1899. 

 ^ United States patent No. 706744, June 6, 1902. 

 « United States patent No. 727.331, April 9, 1903. 

 f Austin, Bulletin of the Bureau of Standards, vol. 2, No. 2. 

 ^ Neughschwender, German patent No. 107843, December 13, 1898, and 

 Schaefer, British patent No, 6002, 1899. 



