Differential Winding Working.
A method of working an electro-magnet intermittently, so as to avoid
sparking. The magnet is wound with two coils. One is connected straight
into the circuit, the other is connected in parallel therewith with a
switch inserted. The coils are so connected that when the switch is
closed the two are in opposition, the current going through them in
opposite senses. Thus one overcomes the effect of the other and the
magnet core shows no magnetism, provided the two coils are of equal
resistance and equal number of convolutions or turns.
Fig. 129. DIFFERENTIAL WINDING WORKING OF ELECTRO-MAGNETIC APPARATUS.
Diffusion.
A term properly applied to the varying current density found in
conductors of unequal cross sectional area. In electro-therapeutics it
is applied to the distribution of current as it passes through the human
body. Its density per cross-sectional area varies with the area and
with the other factors.
Diffusion Creep.
When electrodes of an active circuit are immersed in a solution of an
electrolyte, a current passes electrolytically if there is a sufficient
potential difference. The current passes through all parts of the
solution, spreading out of the direct prism connecting or defined by the
electrodes. To this portion of the current the above term is applied. If
the electrodes are small enough in proportion to the distance between
them the current transmission or creep outside of the line becomes the
principal conveyor of the current so that the resistance remains the
same for all distances.
Dimensions and Theory of Dimensions.
The expression of the unitary value of a physical quantity in one or
more of the units of length (L), time (T) and mass (M) is termed the
dimensions of such quantity. Thus the dimension or dimensions of a
distance is simply L; of an angle, expressible by dividing the arc by
the radius is L/L; of a velocity, expressible by distance divided by
time--L/T; of acceleration, which is velocity acquired in a unit of
time, and is therefore expressible by velocity divided by time--L/T/T or
L/T2; of momentum, which is the product of mass into velocity--M*L/T; of
kinetic energy taken as the product of mass into the square of
velocity--M*(L2/T2); of potential energy taken as the product of mass
into acceleration into space-M*(L/T2)*L reducing to M*(L2/T2). The
theory is based on three fundamental units and embraces all electric
quantities. The simple units generally taken are the gram, centimeter
and second and the dimensions of the fundamental compound units are
expressed in terms of these three, forming the centimeter-gram-second or
C. G. S. system of units. Unless otherwise expressed or implied the
letters L, M and T, may be taken to indicate centimeter, gram and second
respectively. It is obvious that very complicated expressions of
dimensions may be built up, and that a mathematical expression of
unnamed quantities may be arrived at. Dimensions in their application by
these symbols are subject to the laws of algebra. They were invented by
Fourier and were brought into prominence by J. Clerk Maxwell. Another
excellent definition reads as follows: "By the dimensions of a physical
quantity we mean the quantities and powers of quantities, involved in
the measurement of it." (W. T. A. Emtage.)