The Johnsen-Rahbek effect is briefly described and the existing theories of the phenomenon are discussed. A model is derived expressing the attractive force between the contacting materials as a function of the voltage applied across them. The model, which takes into account the presence of surface irregularities, is obtained by an assessment of the effect of field emission on the electrostatic capacitor forces between the contacting surfaces. Good agreement with existing experimental results is obtained.

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At the request of the Standards Commnittee this paper has been prepared, the object being to present data relative to the carrying capacity of cables, which will be of value to engineers and users of cables. Especial attention is called to Table I (to be explained later) which gives data relative to the overload capacity of cables which have been working at different percentages of the normal carrying capacity. This question comes up frequently, engineers asking what per cent of overload can be applied to different cables. The per cent of such overload must of course depend upon the per cent of normal load at which the cable has been operating for several hours previous to the application of the overload. It is hoped that the formulas presented will be of value to engineers who have to deal with the subject under consideration.The current rating of an electric cable depends entirely-upon allowable heating, and is often more dependent upon or limited by external conditions than upon conditions intrinsic in the cable itself. On account of the extreme variability of these external conditions, the current rating of any given cable may be reduced to a half or a third, or even a small fraction of its normal rating. The time during which a cable may carry any given per cent overload is shorter than the time during which most other electrical apparatus can carry the same per cent overload. The allowable overload for any given duration of time depends simply upon the rate at which the cable will be heated to its limiting temperature. This is much more dependent upon the properties of the cable itself, than is the ultimate carrying capacity. 325

The following paper is an investigation of the losses produced in a system by the distributing transformers, with a view to determining the cost to the central station of supplying these losses. The cost of the losses occurring in a transformer is of the same order of magnitude as the cost of the transformer itself and should, therefore, receive the same amount of considera tion. If a transformer had a perfect magnetic (iron) circuit and a perfect electric (copper) circuit, no losses would be pro duced if it were placed on a system. Hence the losses which do actually occur come into two general divisions, viz.: (1) Losses due to imperfect iron, and (2) Losses due to imperfect copper.The expense involved may be divided as follows: 1. Elements due to imperfect iron. (a) Iron loss, involving: (A) Consumption of energy in transformer. (B) Station and line capacity to take care of such energy. (b) Magnetizing current, involving: (A) Copper loss in generator and line. (B) Generator and line capacity to take care of this magnetizing current. 2. Elements due to imperfect copper. (a) Copper loss, involving: (A) Consumption of energy in transformer. (B) Station and line capacity to take care of such energy. (b) Fluctuating secondary voltage, causing shortening of life of lamps, and perhaps dissatisfaction to consumers.