Skin effect and proximity effect in tubular conductors

The effective a-c. resistance of tubular conductors is required to be predetermined by designers, for radio installations, for large underground cables with non-magnetic cores, and for electric furnace circuits. (See Examples I to IV). For the above purpose, sets of curves are given in this paper. The skin effect ratio for isolated tubes is shown in Fig. 1. For stranded conductors, the resistance must be multiplied also by a ratio for the spirality effect, as is approximately indicated in Fig. 2. When the return conductor is near, a ratio for the proximity effect, as indicated in Fig. 3, is also to be used. A calculation for the proximity effect ratio for thin tubes is made, and the results are compared with tests in Fig. 3. Some of the requirements for future research work on skin effect are discussed. The conclusion is expressed that it seems scarcely worth while providing a non-magnetic core with a 2,000,000 cir. mil, 25-cycle cable in order to reduce the skin effect, but with the other cases considered, the tubular form seems very advantageous.