by the residual magnetism in the material after the magnetizing force has been wholly withdrawn. If it is gradually made to decrease from a positive maximum to zero, then reversed and increased to a negative maximum, now decreased to zero, again reversed, and lastly, increased to the positive maximum from which it started, the curves between induction B and magnetizing force H will not be coincident, but will form a loop which represents a complete cycle of magnetization. Its area is a measure of the work wasted or transformed into useless heat in carrying the iron through the cycle. With hard iron, and particularly steel, the loop has a large area, indicating a large waste of energy. The tendency of the changes of magnetization to lag behind the changes of magnetizing force is called magnetic hysteresis, and it may be regarded as a kind of internal or molecular magnetic friction, by reason of which alternate magnetizations cause the iron to grow hot. The effect of hysteresis is to prevent any simple relation existing between H and B or H and I; hence, to define μ, the way in which the iron has been treated must be known. The area of the loop, viz. H. dB approximately that of a rectangle, the height of which is double the remanence and the breadth double the coercive force. The curves forming the loop, which is usually called the BH curve of hysteresis, can be conveniently determined by the step-by-step ballistic method. = Apparatus.-The same as that for the Permeability Test for Rings (Ballistically). Observations.—(1) Connect up the apparatus as shown in Fig. 71, and adjust the galvanometer needle to zero. With K open and K, closed, take the specimen through two or three complete cycles of magnetization in the way indicated above, stopping at the positive maximum, but still allowing this positive maximum current to continue to flow. (2) With S in its last position in (1), and K, K, closed, quickly increase 1⁄2 so as to suddenly reduce the current by a suitable amount to a lower value A, amps. Note this and the throw d1 divisions. (3) From this new current value A, take the specimen through one complete cycle, ending at the same value A,. (4) Repeat (2) and (3) alternately for about five equal decrements of current down to zero. (5) Reverse the direction of magnetizing current by means of S, and with K, K, closed, obtain throws for about ten sudden equal increments of current, with one cycle after each throw, up to the negative maximum, as indicated in (3). (6) Repeat (2)-(4), with S in position used in (5). (7) Reverse the current, and repeat (5) up to the positive maximum. NOTE.-Care must be taken that no error is made in measuring the throw at any step, as it will cause an error in all future readings. To avoid this, three throws should be taken at each current, and the mean noted. The amount of magnetism at any stage is given by the algebraical sum of the throws up to that point; hence care must be taken to affix the right sign. Tabulate as follows: : For the various symbols, see experiment on Permeability (Ring Method). (8) Plot the BH curve, reckoning axes north and east positive and south and west negative, and having B on the ordinates. Inferences.-State clearly the inferences you can draw from the results of the above experiment. Find the residual magnetism, as a percentage of the induced, also the watts wasted in the specimen, if the work done per cubic centimetre of the metal H.dB S H per cycle. If H and B are in C.G.S. units, W will be in ergs per cubic centimetre. If N WN = number of cycles per second, then watts wasted = The actual method of plotting a B H curve of magnetic hysteresis from the experimental figures obtained may not perhaps be at first sight perfectly obvious to every student. The method procedure, therefore, is as follows :— Take any point A as a temporary origin, from which draw the two rectangular axes, AC and AD. Arrange so that the ordinates AC represent the figures in the column headed B of the table, and the abscissæ AD those in the column under H. Now, on plotting in the usual way the first half of the two columns up to the negative maximum for H, we obtain the curve APS,A,; next plotting the return set of figures, or last half of these columns, we get the curve A,P,SA. These two curves will probably not coincide at A. However, together they form a complete cycle of magnetization. Next halve AC, the vertical distance between AA,, drawing the horizontal line XOX1. Similarly bisect AD, the horizontal distance between AA, drawing the vertical line YOY,, intersecting XOX, in the point O, which is the true origin of the cyclic curve. Now renumber the abscissæ XX, either way from O with the same scale as AD; also renumber the ordinates YY, each way from O with the same scale as AD. Then D -H Y P Y -B FIG. 74. OX and OX, will represent +H and H respectively, while OY and OY, will represent +B and B respectively. OP, the induction which remains after the magnetizing force H is gradually reduced from its maximum to O, is called the remanence, or retentiveness, and the magnetizing force OS, required to annul the induction is called the coercive force of the material. 92. Measurement of Magnetic Hysteresis (Magnetometer Method). Introduction. This method only differs from the corresponding one for measuring magnetic permeability solely in the manner in which the magnetizing current is varied. It forms a convenient and accurate means of measuring the magnetic hysteresis of any material, and is eminently suited for testing those materials which possess considerable magnetic. lag and creeping. The remarks made in connection with the permeability test by the magnetometer method (pp. 146-153) apply in the present instance precisely, and therefore the reader is referred to that test for all details of the method. Apparatus. Same as in the above-named method. Observations. (1) Carry out Obs. (1)-(8) of that test exactly as there indicated, but do not break the magnetizing circuit when the maximum (which will be termed the positive) has been reached. (2) Reduce this current in about three or four steps to O, noting its value A and the steady scale-deflection a at each. N.B. The deflection a, when A = O, gives the remanence, or residual magnetism, in the specimen. (3) Reverse at K and close it, repeating Obs. (7) up to the negative maximum. (4) Repeat (2) above, down to zero. (5) Reverse at K, and close it, repeating Obs. (7) again up to the positive maximum, thereby completing one cycle of magnetization. (6) Calculate the values of B and H, and tabulate as shown. (7) Plot the BH curve of magnetic hysteresis, having B as ordinates and H as abscissæ, paying every regard to "sign," and reckoning axes north and east as positive and south and west as negative. Inferences.-State any inference you can draw from the above test. Find the watts wasted in the specimen and the residual magnetism as a percentage of the induced. 93. Capacity (Absolute and Comparative Determinations). Introductory.-Without in any way considering the actual practical methods of constructing condensers, in connection with which we are most frequently accustomed to speak of capacity, the following remarks, which apply generally to all such apparatus, are perhaps necessary to impress upon the reader the various terms, phrases, and phenomena met with in this class of work. A condenser may be defined as an arrangement of two conductors separated by an insulator; the former are called the coatings, and the latter the dielectric, of the condenser. If the poles of a source of E.M.F. be connected to the respective coatings, a P.D., V, will be acting across them, and a quantity, Q, of electricity will flow into the condenser. It will then be found that, if there is no leakage, Q ∞ V, or = C (a constant), which is called the capacity of the condenser. Hence the capacity of any condenser is the quantity of electricity required to be given to either coating in order to produce unit potential difference between the coatings. The unit of capacity is called a farad, and = 109 C.G.S. absolute units of capacity. A condenser has a capacity of 1 farad when a P.D. of 1 volt between its coatings charges each of them with I coulomb. we see that the dimensions of capacity are of the order of time resistance' velocity' L L' length magnetic system of units. in the electro T This "absolute unit" is far too large to deal with, and even the farad, which, as we see, is only absolute unit, is a great deal too large for practical purposes, so that a smaller unit = one-millionth part of a farad, and called, consequently, a microfarad, is universally adopted. Hence I microfarad 10-15 C.G S. unit. When a condenser is charged, some time elapses before its terminal P.D. rises to its maximum value, or, in other words, before the whole charge is absorbed or gets in, which does not occur immediately. This "soaking in," so to speak, is due to surface action in the dielectric. Similarly, on discharging, the whole of the previous charge is not removed immediately ; some has soaked in giving rise to what is called a residual charge, part of which can be obtained on again |