PAPERS OX CHEMISTRY AND PHYSICS 299 



A CABBON FILM HIGH RESISTANCE; ITS CON- 

 STRUCTION AND CHARACTERISTICS. 



A. J. McMaster, University of Illinois 



The problem of obtaining at low cost a satisfactory 

 high electrical resistance, that is from 0.1 to 100 meg- 

 ohms, for use in experimental work is one with which 

 many laboratories of physics have been confronted. The 

 most common means of meeting this problem has been 

 to use a graphite line on ground glass or hard rubber 

 with some sort of pressure contacts as terminals. How- 

 ever in Dec. 1902, Prof. A. C. Longden described in a 

 paper in the Physical Review* a new form of carbon re- 

 sistance in which he used as a resistance material a 

 film of smoke deposited from a flame upon a strip of 

 glass. The terminals which he devised, and which are 

 very satisfactory, consist of a film of silver deposited 

 chemically on each end of the strip. The capillary at- 

 traction between the glass and the silvering ' solution 

 causes the end of the film to be almost infinitely thin. 

 The carbon film, when deposited over the central por- 

 tion of the glass and onto the silvered ends, forms a 

 very smooth and satisfactory contact. A small copper 

 wire is copper plated to each of the silvered tips to com- 

 plete the terminals. By treating the carbon film with al- 

 cohol vapor, it is sufficiently hardened to permit a thin 

 layer of paraffin or shellac to be flowed over it. A num- 

 ber of such slides may be mounted in a dry wood or 

 hard rubber case which is provided with binding posts. 



It is this same type of carbon film resistance, but in a 

 new and perhaps a little more convenient form and 

 mounting, that has been the subject of the present in- 

 vestigation. The construction of the resistance is shown 

 in detail in Fig. 1. Instead of a glass slide, a small soft 

 glass tube, sealed at each end, is used, in which there is 

 a slight constriction near each end. The silver copper 

 terminals are constructed as prescribed by Longden. The 

 silver is deposited chemically by Brashear's process for 

 silvering glass, and the copper is deposited electrolytic- 



* Phys. Rev. Vol. XV.— No. 



