THE SUSCEPTIBILITY-TEMPERATURE APPARATUS This instrument is a form of an inductive method of

measuring the magnetic susceptibility adopted to investigate the change of susceptibility with temperature of rock

samples or rock minerals. Its main principle, which has been (6)

used elsewhere, is the fact that a rock, when placed in an alternating magnetic field, becomes an alternating magnet that produces a secondary alternating field proportional to its magnetic susceptibility. The apparatus used here is a modification of a previous one which has been described by

(16) Manley.

The circuit employed is illustrated diagrammatically in fig.(15). Two balanced pick up coils PI and P2, in series opposition, are fixed coaxially inside a pair of big Helmholtz coils which is energized by an alternating current and

produces the magnetizing field. In the absence of the rock specimen, the resultant e.m.f. in the pick up coils is about zero, since they have the same mutual inductance with the

Helmholtz coils. When the rock specimen is introduced near to the pick up coils, it produces a bigger e.m.f. in the nearer coil than in the other and the differential e.m.f. is a direct measure of the magnetic moment of the specimen. This e.m.f.

is balanced and measured on a potentiometer which is fed by a standard e.m.f. produced by a small auxiliary coil (the neutralizer) P3 wound on P2. Since this standard e.m.f. is

H

AMPLIFIER

VIBR. GALV.

H

MILLIAMMETER.

"'MAINS

50 WATT 'AMPLI FIEf\

OSCILLATOR

FIG. 15

==771

I-

I

I

HH.: HELMHOLTZ COILS P2 PICK UP COILS

P3 NEUTRALIZER COIL F : FURNACE

W : WATER JACKET T : THERMO-COUPLE

S SPECIMEN C :CARRIAGE G : GROOVE

B :BRIDGE

Fl G. 1 6

21.

proportional to the exciting field, the balance on the potentiometer gives a measure of the ratio of the induced moment to the field, i.e. a measure of KV where K is the susceptibility and V the volume of the specimen. To detect the balance, a three stage, low noise amplifier is used and whose output stage is coupled to a Campbell Vibration

galvanometer tuned to the frequency of the exciting field.

To study the variation with temperature of the susceptibil-ity, the specimen is placed inside a non-magnetic, non-inductive

electric furnace which is situated inside the Helmholtz coils, in front of the pick up coils. The furnace draws its heating current from a 110 V., D.C. supply through a rheostat and is surrounded by a water jacket. This is necessary to reduce heat radiation to the pick up coils which would be warped by any slight change of their temperature. The temperature of the furnace is measured by means of a non-magnetic thermocouple made of Platinum-Platinum Rhodium and connected to an

"Electroflon temperature recorder.

Details and improvements of the apparatus

(a) The Helmholtz coils and detecting coils

gee

The Helmholtz coils consist of 50 turns each, of 0.20 cm.

cotton covered copper wire wound with a mean radius of 32 cms.

The wooden formers are accurately spaced by rods with

insulating nuts at their ends. To hold these formers rigid, they rest on a big wooden stand and are fixed from outside to

two big supports made of thick wood. These supports are, in turn, fixed to the floor to attain maximum rigidity of the system without the use of metal. This was done because it was found that any mechanical vibration or very small displace- ment would alter the electric balance of the system.

The pick up coils P1 and P2 are rigidly attached to one of the Helmholtz coil formers by means of a tufnol spindle through their centres with plastic spacers separating them and the whole column is clamped between two big tufnol nuts. The pick up coils were balanced to the nearest turn as regard

their mutual inductances with the Helmholtz coils, in the same way as described above in the spinning magnetometer.

On the coil P2 there is a third coil P3, consisting of 10 turns of wire which, through its mutual inductance with

the Helmholtz coils, provides the e.m.f, feeding the potentiometer.

Although the pick up coils were balanced to the nearest turn, there is, in the absence of any specimen, a small

differential e.m.f. giving rise to a "zero" reading on the potentiometer. It is the difference between the balancing reading, when the specimen is present, and this zero reading,

that is a measure of the magnetic susceptibility of the rock sample.

The pick up coils are connected, through thin parallel wires to a bank of variable condensers situated outside the Helmholtz coils, the condensers being used to bring the two

23.

e.m.f.s in the coils into phase with each other. These condensers are sensitive to "hand capacity" and so are

manipulated by long plastic rods fitted into the plastic heads of the condensers.

In table (3) the actual details of the coils are given.

Table (3)

The main source of trouble in the original apparatus was that the zero reading was not constant but irregularly

changing. It might be due to slight changes of room tempera- ture, mechanical vibrations or to some influence of the

(16)

furnace temperature. As a result, Manley was unable to

trace accurately the susceptibility-temperature curves of rock specimens. To overcome this, it became necessary to observe the zero reading just before or after any reading taken with the specimen at any temperature.

In its present form, the furnace rests on a wooden ft carriage" to which it is firmly secured. The carriage has two parallel wooden "tongues" on its base and these can slide in two parallel grooves cut in a horizontal wooden bridge.

The bridge is firmly fixed to the floor with its length

perpendicular to the axis of the Helmholtz coils and extending outside them.

When the furnace, with the specimen inside, is moved outside the Helmholtz coils a zero reading can be taken, whereas when it is returned in front of the pick up coils it gives rise to a second reading corresponding to the presence of the specimen. In order to have the furnace in exactly the same position in front of the pick up coils, for each reading, there are two wooden stops fixed on the bridge between which the furnace can slide. By moving the furnace until its carriage is against either the inner stop or the outer stop, standard positions relative to the pick up coil system are achieved.

No ferromagnetic material was used in fixing the different parts of the system and, whenever possible, the minimum amount of brass screws were used so as not to have any serious or detectable eddy current effects on the pick up coils. A

photograph of the whole system is shown in fig. (17).

(c) The energizing current

In the original apparatus, the energizing current for the

FIG. 17

Helmholtz coils was drawn from the mains through a 12 V transformer. This was found to be unsatisfactory since any slight change in the frequency of the mains gave a troublesome interference and balancing the apparatus could not be done accurately. Also, extraneous magnetic fields resulted in poor balancing. Accordingly, a frequency of 65 cycles per second produced by an oscillator was used. This was a

standard DAWE oscillator, type 4000, fitted with a time base to adjust the frequency employed. The output of the oscillator was coupled to the input of a 50 watt VORTEXION amplifier to produce the necessary energizing current. The output was Connected in series with a milliammeter and a small slide resistance to the Helmholtz coils. The output impedance of

the amplifier was made to match that of the Helmholtz coils through a matching transformer fitted to the amplifier. The energizing current was kept at a constant value of 0.415 amps.

(R.M.S.) to provide an alternating magnetizing field of the same order as the earthts field (0.5 gauss),since the magnetic

(11) susceptibility is a function of the exciting field. The vibration galvanometer was tuned, consequently, to this

frequency. A circuit diagram of the whole equipment is shown in fig. (15).

Experimental procedure

The amplifier, used to detect the balance, and the

exciting current supply were turned on and allowed to warm up

26.

until the energizing current reached its steady value of

0.415 amps. Starting with a small amplification, the condensers and the potentiometer were altered, in turn, until, with

maximum amplification balance was reached - the furnace being at the "in" position. The specimen was then slowly pushed into the furnace to its optimum position, with respect to the pick up coils,where the specimen produced a maximum deflection on the vibration galvanometer. The two ends of the furnace were then closed by plugs made of plaster of Paris and through one of them the thermocouple was introduced till its end

touched the specimen.

The furnace was then moved to the "out" position and balance attained to give a zero reading on the potentiometer.

Then it was slowly moved to the "in" position and a second balancing reading observed corresponding to the presence of the specimen. This was repeated several times, at room temperature, as a check of the stability of the apparatus.

The furnace current was then switched on and the cooling water circulation started and the former was adjusted to give a constant slow rate of heating as observed on the temperature recorder. At intervals of 10-150C the furnace was moved in and balance obtained and immediately the temperature was read.

This was followed by moving the furnace out to note the corresponding zero reading.

In this way, continuous readings of the magnetic

susceptibility could be observed every 4 minutes at intervals

of about 10-15°C. The heating of the specimen to the Curie point (600°C approx.) could take half an hour, but in practice it was arranged to take a total of 3 hours for heating and cooling, both being approximately at the same rate. This was done for the double purpose of enabling as many readings as possible to be taken, so as to have a well defined shape of the K-T curve, and to enable the rock to attain the temperature of the furnace. If the heating was done quickly, the temper-ature of the rock would be less than that of the furnace, while during cooling it would be more due to the bigger thermal

capacity of the rock over that of the thermocouple.

After making these modifications and before using the apparatus for actual observations, the changes in the zero reading were investigated by two tests.

In one, and at room temperatures balance on the potentio-meter was obtained for the two standard positions of the

furnace without a specimen. The mean difference, of a set of observations, between the two corresponding readings, was very small (less than 4 potentiometer units). This indicated that the furnace and its thermocouple had no appreciable

magnetic material in them.

In the second test, the furnace was heated slowly to 60000 and allowed to cool down to room temperature, without a specimen, and readings recorded during the run. With the furnace in the "out" position, the zero reading changes slowly but irregularly during heating and cooling. Similarly, the

28.

readings with the furnace "in", showed similar changes but the difference between them was small and constant so that, for practical purposes, the furnace could be regarded as non-magnetic.

C. THE THERMOMAGNETIC SEPARATOR