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Volume 28, 1895
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Art. XIX.—A Comparison of the Magnetic Screening produced by Different Metals.

[Read before the Philosophical Institute of Canterbury, 6th November, 1895.]

When a conductor is placed in a varying magnetic field the currents induced in it tend to keep the field constant. If the field varies slowly the effect is slight; but in fields produced by rapidly-alternating currents the “screening” is very marked.

In these experiments the fields were produced by leydenjar discharges. Magnetized steel needles were used as “detectors” (Rutheriord, Trans. N.Z. Inst., 1894, p. 488). Magnetized steel needles are much more suitable for this purpose than unmagnetized, for a field too weak to magnetize a needle to any appreciable extent is capable of producing considerable demagnetization.

The discharge passed through a coil of several turns, inside which a magnetized steel needle was placed, and in whichever direction the needle lay it was partially demagnetized; but the demagnetization was greater when the needle was placed in that direction in which the field, due to the first semi-oscillation of the discharge, demagnetized it. Hereafter this direction will be referred to as “direction a”; the direction in which the field due to first semi-oscillation tended to magnetize the needle as “direction b.

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If a metallic screen was placed inside the coil, so as to surround the needle, the demagnetization produced by the discharge was less.

The screening depends on the thickness and on the conductivity of the screen, and on the frequency of the discharge.

The condenser, which consisted of ordinary 40oz. leydenjars, was charged by a Voss influence machine, and connected in series with several coils wound on glass tube of 21mm. diameter. In the circuit was a spark-gap, of length 3.7mm., and the diameter of the knobs was 2.8cm.; hence the potential at discharge was about 43.5 electrostatic units, or 13,000 volts. (J. J. Thomson, “Recent Researches,” p. 77.) It was found that the effect of the discharge varied less with, this length of spark than with a shorter spark.

The needles used were of glass-hard pianoforte-steel wire. They were magnetized to saturation by placing them in a coil (of 191 turns, and of length 9.9cm.) through which passed a current of from six to ten ampères, produced by a Grove's battery or by an accumulator. The needles, for convenience in handling, were sealed in fine glass tubes.

The needle, after being magnetized, was placed at a distance of 102.5cm. from the needle of a magnetometer. The magnetometer readings were taken by the ordinary lamp-and-scale method. The needle was then placed in one of the coils in the leyden-jar circuit and a discharge was passed. The needle was again tested by the magnetometer and the new reading noted. Great care had to be taken in replacing the needle, and in order to avoid error the magnetometer was kept in a fixed position throughout the experiments, while the needle was placed in a groove cut in a bar attached to a stand, which was itself screwed down to the table on which the instrument stood.

The screens were metal cylinders, which could be placed inside the coils. They were formed by winding thin sheets of metals on glass tubes.

Great care had to be taken to make the contact at the junction good; if there is merely touching contact the screening is much reduced.

The screening produced by different metals was compared as follows: Tubes were wound with different numbers of layers of tinfoil. The reductions in the deflection produced on the magnetized needle when surrounded by these and placed in a certain coil (of 2.04 turns per centimetre) were observed. Similar experiments were made with cylinders of other metals, and the numbers of layers of tinfoil producing the same effect were calculated by interpolation.

The diameter of the tubes on which the metals were wound was 14.4mm., and the length of metal 16cm. The

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tinfoil was pasted on to the tubes. In the case of the other metals used—lead, zinc, silver, and copper—the junction was made by soldering. The only other metal that could be obtained in thin sheets was aluminium, and the difficulty in making a junction precluded its use. Unfortunately, only one thickness of each of the metals could be obtained, and that was so great that only one layer could be used.

The thickness of the metals somewhat restricted the scope of these experiments, and rendered the method less sensitive than it would otherwise have been; while, in order to get sufficient reduction of deflection with the needle placed in direction b, it was necessary to use as a condenser four leydenjars arranged in parallel. The thicknesses of the metals used were—

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Copper 0.0134 Millimenters.
Silver 0.0108 "
Zinc 0.0430 "
Lead 0.0957 "
Tinfoil 0.0116 "

In the first set of experiments the condenser consisted of two leyden-jars arranged in parallel. The length of the needle used was 60.4mm., and it was placed in direction a, for with the needle placed in direction b the copper, zinc, and silver were thick enough to screen off all effect, while in the case of the lead the screening was at least 95 per cent. The observations made are here tabulated:—

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Number of Layers of Tinfoil. Original Deflection. Reduced Deflection. Reduction.
0 201 102 99
5 201 129½ 71½
6 201 134 67
8 201 141 60
9 201 144 57
10 201 147 54
Metal. Origninal. Deflection. Reduced Deflection. Reduction.
Silver 201 141 60
Copper 201 144½ 56½
Zinc 201 143 57
Lead 201 133 68

The results obtained from these observations were that the silver produced the same screening as 8 layers of tinfoil; that the copper was equivalent to 9.2 layers, the zinc to 8.8 layers, and the lead to 5.8 layers.

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A second set of experiments was made with four leyden-jars, arranged in parallel, placed in the circuit, and the needle placed in direction b. The length of the needle used in this set of experiments was 74.5mm. The observations are here tabulated:—

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Number of Layers of Tinfoil. Original Deflection. Reduced Deflection. Reduction.
0 252 155½ 96½
5 252 225½ 26½
6 252 231 21
7 252 235 17
8 252 239 13
9 252 243 9
10 252 246½
11 252 250 2
12 252 252 0
Metal. Original Deflection. Reduced Deflection. Reduction.
Silver 252 241 11
Copper 252 244 8
Zinc 252 243 9
Lead 252 231½ 20½

The results of this set of experiments were that the silver was equivalent to 8.5. layers of tinfoil, the copper to 9.3 layers, the zinc to 9 layers, and the lead to 6.1 layers.

These experiments show that the thicknesses of different metals required to produce the same screening are proportional to the specific resistances of the metals.

In the next table the equivalent thicknesses obtained by experiment are compared with those calculated by taking the thickness proportional to the specific resistance:—

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Equivalent Number of Layers of Tinfoil.
Metal. Deduced from First Set of Experiments. Deduced from Second Set. Deduced by taking Thickness Proportional to Sp. R.
Silver 8.0 8.5 8.2
Copper 9.2 9.3 9.5
Zinc 8.8 9.0 8.7
Lead 5.8 6.1 5.6

Professor J. J. Thomson, in a paper on “The Resistance of Electrolytes to the Passage of very Rapidly - alternating

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Currents” (Proceedings, Royal Society, 17th January, 1889, vol. xlv.), investigates mathematically the screening produced by a conducting-plate, and shows that it is proportional to the thickness, and that if plates of different metals produce the same screening their thicknesses will be proportional to their specific resistances.

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The screening mentioned by Professor Thomson is measured by the decrdase of electro-motive force, while in these experiments the screening is measured by the decrease of magnetic force; and it is possible to have very considerable screening of electro-motive force and at the same time very little screening of magnetic force, for the magnetic field may be large though its rate of change is small. Professor Thomson (see paper mentioned above) found that a thickness of 1/1700 of a centimetre of Dutch metal screened off all electromotive force, and the thinnest films of metal he could obtain screened off all effect.