An analysis is made of the shielding effect of eddy currents on the flux in the interior of cores of cylindrical or flat sheet material. It is shown that the counter voltage of self inductance of an iron-cored coil is due only to the component of flux in the core which is in phase with the flux at the surface of the core.Expressions are obtained and curves plotted showing the variations of inductance of a coil with frequency, or with the conductivity and permeability of the core material. Sample calculations and some experimental results are given. The results show that the inductances at high frequencies are actually less than the predicted values, which leads to the suspicion that some factor other than eddy currents causes the flux in the interior of the cores to decrease with increasing frequency. INTRODUCTIONT IS well known that if a sinusoidal electromotive force is impressed upon a cylindrical or sheet cond'uctor, the current density will not be uniform throughout the body of the conductor, but will vary from the surface to the center in a manner which depends upon the frequency of the impressed e.m.f. and upon the magnetic permeability and electrical conductivity of the conductor material. Similarly if a sinusoidal electromotive force is impressed upon a coil enclosing a core of cylindrical or sheet material, the magnetic flux density within the core will vary in the same manner and with the same quantities as the current density in the electrical case.These phenomena are probably best known as 'skin effect" in the case of conductors, and as the "shielding effect of eddy currents" in the case of cores. The equations for current density or flux density are identical in form for either wires or sheets, whence the current

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the rotor revolves only 60 times per minute, the speed required for a split-second hand.Around the inner edge of the rotor ring are 122 teeth ; inside this is a second rotating iron ring of still smaller diameter with 120 teeth around its outer edge. In this second reduction of speed the first rotor be comes the stator for the second, but while the first is running forward 60 revolutions, the second is run ning only one, so that its net progress forward is one revolution per minute. This is the speed desired for the second hand.Attached to the second hand rotor is a permanent horseshoe magnet which creates a new rotating field in 118 iron teeth driving still another ring with 120 teeth at the rate of one revolution per hour. This runs the minute hand. Attached to the minute hand rotor is a second permanent magnet creating a rotating field in 22 teeth and driving a 24-tooth rotor one revolution every 12 hours for the hour hand. The entire operation may be explained by saying that each rotor travels only two teeth for each revolution of its magnetic field. Obviously by changing the number of teeth, any desired speed may be obtained.A new relationship connecting the opencircuit remanence of a permanent magnet with the factors which determine its value has been found. This relationship, shown by Fig. 2 of the following article, appears to be general and valid for all kinds of magnet steel. Also, criterions of magnetic quality are presented/ including a nomo gram chart for permanent magnet design.

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