ACTIVATION OF ELECTRICAL CONTACTS BY ORGANIC VAPORS 789 



cm all arcs were of the anode type with the characteristic pit on the 

 anode and a roughened spatter of metal on the cathode. It is clear from 

 these tests that active arcs at silver surfaces are of the anode type if the 

 layer of carbonaceous material responsible for activation is not heavy 

 enough to permit an arc to strike at a separation greater than 2.5 X 10~^ 

 cm, but that they are of the cathode type when the layer is sufficiently 

 thick to permit arcs at 5 X lO""* cm. The electrode separation at which 

 an arc takes place determines the character of the arc* The critical 

 distance for silver surfaces lies between 2.5 and 5 X 10~^ cm. Erosion 

 at active silver surfaces on closure must be predominantly from the 

 anode unless the layer of activating carbonaceous material is so heavy 

 that arcs strike when the electrode separation is greater than 2.5 X 10~^ 

 cm. 



After long continued operation at very high pressures of activating 

 vapor it is sometimes, but not always, found that arcs on closure result 

 in erosion that is chiefly from the cathode. The conclusion drawn from 

 these measurements is that the striking distance at active surfaces on 

 closure at low voltages can sometimes, with considerable difficulty, be 

 made greater than 2.5 X 10~'* cm. Unless great pains are taken to keep 

 surfaces very heavily carbonized, the striking distance on closure at 

 active surfaces at low voltages is of the order of 2.5 X 10~^ cm or less. 

 On closure at voltages that give air breakdown, the erosion of silver is 

 predominantly from the cathode whether the surfaces are active or 

 inactive, because the minimum distance for air breakdown (15 X 10~* 

 cm) is much above the critical distance for silver. 



On breaking active silver contacts in an inductive circuit, erosion is 

 chiefly from the anode unless the arc lasts long enough for the electrode 

 separation to exceed the critical distance of 3 or 4 X lO""* cm. During 

 the time an arc persists at distances greater than this, the loss is pre- 

 dominantly from the cathode. For velocities typical of a U-type relay, 

 the critical distance may be reached in 10 or 20 microseconds, and equal 

 erosion may be attained in a time of the order of 40 microseconds. If 

 the partial pressure of activating vapor is very high and the surfaces 

 unusually heavily carbonized, much of the eroded metal will be lost. 

 Under more usual conditions of lower vapor pressures, most of the eroded 

 metal is transferred to the opposite electrode. Thus there may be a criti- 

 cal arc duration for which the erosion of each silver electrode is nearly 



* Similar tests were carried out upon polished gold surfaces upon which sparse 

 layers of carbon particles had been dusted. For particles of maximum diameter 

 2.5 X 10-^ cm all arcs were found by microscopic examination of the electrodes 

 to be anode arcs, and for particles in the range of diameters from 4 to 5 X 10~* 

 cm all arcs were found to be of the cathode type. These results are identical with 

 those found for silver. 



