500 Mr. F. B. Fawcett on Standard High Resistances. 



wire. In this case, while there is no manifestation of electro- 

 static force, there is strong alternating magnetic force, which 

 in the space outside the globe is as it' from an ideal infinitesi- 

 mal magnet with alternating magnetization, placed at the 

 centre of the globe and with its magnetic axis in our axial line. 



LI. On Standard High Resistances. 

 By F. B. Fawcett, University College, Bristol*. 



f j HEBE are at the present time two principal forms of 

 high resistance in use. That made of insulated wire, 

 which is reliable but costly, and the carbon line on ebonite 

 which is neither. The purpose of this paper is to describe a 

 method of constructing high resistances which are both cheap 

 and reliable. 



Metal in some form seems to be the only solid conductor 

 of which the resistance remains constant over long periods, 

 and I have therefore examined the properties of the thin 

 metallic films which, as is well known, may be deposited in 

 vacuo from a metallic kathode on surfaces in its neighbour- 

 hood. By making the kathode in the form of a grid of 

 several wires stretched parallel to one another and in the same 

 plane an even deposit of the metal of the wires may be obtained 

 on a strip of glass placed parallel to the grid ; moreover, if 

 the wires are of two different metals, films of what seem to 

 be alloys of these metals may be obtained. Gold and pla- 

 tinum, from their chemical stability, are very suitable for the 

 purpose, and I have therefore used these two substances in 

 most of my later experiments ; the homogeneity of the film 

 being further increased by twisting the gold and platinum 

 wires tightly together and building up the grid of these 

 twists. The results to be described refer to gold -platinum 

 films exclusively. 



When a film is first deposited its resistance alters at a 

 rapidly decreasing rate ; the alteration continuing for many 

 months. There seem to be two causes at work in this — dis- 

 solved gas and molecular rearrangement in the film. If a 

 film which has been exposed to air be placed in a high 

 vacuum the resistance rapidly falls — the change amounting 

 to 3 per cent, in one of my experiments. Prolonged heating 

 also brings down the resistance. This is probably partly 

 due to expulsion of gas ; but that molecular rearrangement 

 occurs is shown by the marked hardening of the film which 

 accompanies it. Before heating the metal may be much 



* Communicated by the Author, having been read at the Bristol 

 meeting of the British Association. 



