In the course of applying the new technique to the analysis of biological substances, a group working under Miles A. McLennan in the Bio-electronics Section of the Aero Medical Laboratory at the Wright Air Development Center has designed a simple version of the magnetic-resonance spectrometer that amateurs can make at home. It should serve not only as an introduction to an interesting new field of experimental physics but should also make an attractive classroom demonstration or science-fair project.

According to the "classical" theory of physics, all elementary particles of matter spin on their axes like tops, and those that have an electric charge (e.g., electrons and protons) generate magnetic fields. (The classical picture has now been superseded by the quantum-mechanical view, but it will suffice for the purposes of this discussion.) Particles bound in atoms and in molecules not only spin but also move on orbits. This motion adds to the field generated by the spin. The fields of neighboring particles merge; depending on the structure of the atoms or molecules and on the direction in which the magnetic forces point, the fields tend to cancel in some cases and to reinforce in others. In consequence all atoms and molecules are characterized by unique patterns of interacting magnetic forces.

What will happen to these tiny magnets if they are subjected to the influence of an external magnetic field? It was this question that led to the development of the new technique. In the case of the single-proton nuclei of the hydrogen atoms of water, the magnetic axes normally point in random directions. It might therefore be supposed that an external field would cause the proton axes to line up in the direction of the field. This, however, does not happen. Instead the field causes the protons to precess, or wobble like a spinning top that has been tipped from the vertical. We might say that each particle now has two axes, one about which it spins and the other about which it precesses. The axes on which the particles precess line up with the external field, but attempts to align the axes on which they spin get nowhere. Increasing the strength of the external field merely causes the particles to precess faster. In fact, the rate of precession varies in proportion to the field strength and is equal to the intensity of the field (expressed in gauss) multiplied by 4,228.5. Thus when a sample of water is placed between the poles of a typical magnetron magnet with a field strength of 1,450 gauss, the hydrogen nuclei precess at the rate of 6,181,325 revolutions per second.

It is possible to disturb the particles, however. They can even be flipped over so their "north" and "south" poles are reversed. This is accomplished by setting up a second external field at right angles to the first and causing it to oscillate or reverse direction precisely in step with the rate at which the particles are precessing. In the case of water in a biasing field of 1,450 gauss the critical frequency is 6.1 megacycles. Energy is absorbed by the particles from the oscillating field during each alternation, just as a tuning fork is set into vibration by the sound waves to which it is resonant. Resonance between the particles and the oscillating field can be established by adjusting either the frequency of the current through the coil or the strength of the biasing field (which determines the rate at which the particles precess). As the oscillating-field frequency approaches resonance the particles absorb energy. As they recede from resonance the borrowed energy is emitted, part being returned to the coil and the remainder being shared with neighboring particles.

In most substances the exchange of energy between the particles and the coil is surprisingly sluggish with respect to the speed of most atomic processes. Some particles respond immediately at resonance, but others require intervals ranging from a few seconds to several minutes. This complicates the design of magnetic-resonance spectrometers because their electrical circuits must be made extremely stable and their output must be observed with the aid of pen recorders.

It turns out, however, that the addition of ferric nitrate to water increases the susceptibility of the particles to the outside field and radically decreases the time required for energy exchange without affecting the rate at which the particles precess. According to the Aero Medical Laboratory group, no completely satisfactory explanation for the action of ferric nitrate has been advanced. It may be that ferric ions in solution decrease the magnetic interaction of the particles and thus render them more susceptible to the influence of external fields. Whatever the explanation, ferric nitrate dissolved in water makes it possible to demonstrate the phenomenon of magnetic resonance with relatively simple apparatus.

The experiment consists of placing a test tube containing the solution of ferric nitrate in the pulsating field of a magnetron magnet and, by means of an oscilloscope, observing the exchange of energy at resonance between the sample and a coil around the test tube which is energized by a vacuum-tube oscillator. The energy absorbed during a single flip of the particles is too small for detection by conventional electronic devices. Hence in this experiment the frequencies are brought in and out of resonance 60 times per second. The frequency of the vacuum-tube oscillator is held constant while the rate of precession is varied by modulating the biasing field of the magnetron magnet. This is accomplished by placing a second coil energized by 60-cycle alternating current between the poles of the magnetron magnet; the flux of this modulating coil alternately reinforces and opposes that of the magnet. The rate of precession varies in proportion. The vacuum-tube oscillator is equipped with two controls, one for adjusting the frequency to the average rate at which the proton axes precess and the other for adjusting the amount of energy fed back from the plate circuit of the vacuum tube to the grid circuit. The latter control regulates the intensity at which the tube oscillates. With this control the oscillator can be put into or out of operation or, when desired, set at the marginal oscillating condition. The modulating coil is wound with a space in the center to admit the test tube, and placed so that its axis is concentric with the biasing field [see illustration above].

With the sample in position the oscillator is turned on and adjusted as closely as possible to 6.1 megacycles, the average frequency at which the protons precess. The feedback control is adjusted for the marginal condition at which oscillations are barely sustained. At this critical point current flowing in the plate circuit of the oscillator tube is highly responsive to changes of energy in the coil around the test tube. The intensity of the plate current is observed by connecting the plate circuit to the vertical terminals of the oscilloscope as shown in the illustration above. A spot of light will appear on the screen, indicating that a fixed value of plate current is flowing. The modulating coil of the biasing magnet is now energized. If the frequency of the oscillator has been adjusted to the average rate of precession, the spot of light will expand into a vertical line, indicating that the plate circuit is responding to energy ex changed between the coil and the particles. The display can be made more interesting by connecting the horizontal plates of the oscilloscope to the 60-cycle power supply which energizes the modulating coil. Typical patterns are shown below.

In the apparatus designed at the Aero Medical Laboratory the magnetic biasing field is supplied by a Type 220A 150 surplus magnetron magnet. The pole faces of the magnet were replaced by soft iron disks 8 1/2 inches in diameter and 7/8 inch thick to provide a field over a large area. For maximum response all protons must precess at the same rate, which means that all must be acted upon uniformly by the modulated biasing field. The intensity of the field will vary with the distance between the pole faces. Hence these must be made parallel and free from surface irregularities. Surplus magnets from magnetrons of the radial-cathode type usually bear a small white dot on the base which gives an approximate figure in gauss for the field strength that may be expected in the air gap. The magnet used in the instrument constructed at the Aero Medical Laboratory is rated at 1,450 gauss. It was modulated by a coil consisting of 20 turns of No. 80 cotton-covered magnet wire wound on a Bakelite tube 1 5/8 inches in outside diameter and 7/8 inch long. Ten turns of the coil are wound at one end of the tube and 10 turns are wound in the same direction at the other end. A hole 5/8 inch in diameter is cut in the center of the coil form to admit the test tube. A second hole 8/8 inch in diameter is made at right angles to the first to admit a length of coaxial cable for linking the oscillator coil to the source of high-frequency current. The modulating coil is energized by the transformer which supplies the tube heaters, and it sweeps the strength of the biasing field 50 gauss above and below its mean value.

The test tube is 12 millimeters in diameter and 75 millimeters long. A two-layer coil of No. 22 enameled magnet wire, consisting of 16 turns per layer, is wound on the straight portion of the tube as close as possible to the closed end. The tube and coil are mounted vertically in the Bakelite form on which the modulating coil is wound.

The circuit construction is conventional. The oscillator is designed around a 6AK5 pentode tube. When used with an oscilloscope of high sensitivity, output from the oscillator may be taken at the junction between the 22,000-ohm resistor and the 200,000-ohm resistor in the plate circuit. With 'scopes of lower sensitivity, such as the Heathkit Model O-10, a single-stage amplifier using a 6AU6 pentode is added as shown in the circuit diagram. A variable capacitor, such as the Hammarlund Type MC140-M, is used for adjusting the frequency of the oscillator. These components are assembled on an aluminum chassis three inches high, five inches wide and six inches long. Input and output connections are made through RG 58/U coaxial cable equipped with UG 290/U and UG 88/U terminals. Power may be taken from any supply capable of delivering 100 milliamperes of direct current at 150 volts to the tube heaters and 60-cycle alternating current at 6.3 volts to the modulating coil.

The test solution is prepared by dissolving .4 gram of ferric nitrate in 100 cubic centimeters of distilled water. Two cubic centimeters of this solution are added to the test tube and placed in the biasing field. Power is applied. After the horizontal-sweep circuit of the oscilloscope has been made synchronous with the 60-cycle modulating voltage, a pattern should appear on the screen. The pattern may resemble a horizontal figure eight, as shown at left in the illustration below. This indicates that the frequency of the oscillator coil lies outside the limits within which the particles are precessing and that resonance is not established. To search for resonance, set the oscillator capacitor for minimum frequency (the plates of the capacitor meshed fully) and adjust the intensity (feedback) control to the point where the oscillator is on the verge of going out of operation. Then increase the frequency slowly while observing the 'scope. It may be necessary to trim the feedback control occasionally to maintain the marginal oscillating condition. The procedure can be simplified with the aid of a short-wave radio receiver. If the receiver is equipped for continuous wave reception, the oscillator signal will be heard as a shrill whistle. If not, it will make a rushing sound, perhaps accompanied by a 60-cycle hum. The receiver is particularly useful in checking the point at which the oscillator goes out of operation when adjusting the feedback control. If the receiver is calibrated, it may be used to calibrate the oscillator. If not, the receiver can be calibrated easily by tuning in on the time signal of Station WWV.

When resonance is established, the display will resemble the center figure below. Usually two peaks appear which are joined at the bottom by loops. This indicates a displacement (phase difference) in the time at which signals arrive at the vertical and horizontal plates of the 'scope. The Heathkit Model O-10 'scope is equipped with a line-sweep switch and a phase control for manipulating the display. When these are properly adjusted, the peaks coincide as shown in the figure at right below.

What does the display mean? The height of the figure is proportional to the number of protons resonating with the oscillator; the width of the figure, to the range through which the particles precess. Accordingly if all of the particles were precessing at precisely the same rate and all flipped over precisely at resonance with the oscillator, the pattern would resemble an inverted "T." The spectrometer could then be said to have perfect resolution. Evidently in this instrument all the particles do not precess at the same frequency. Part of the explanation lies in the interaction of magnetic forces within the test sample. The fields of neighboring protons merge in such a way that some particles are partially shielded from the influence of the outside field. But in this instrument the breadth of the peaks is largely explained by cross-sectional variations in the strength of the biasing field. Particles in regions of high-field intensity precess at higher rates than those in regions where the field is relatively weak. These differences are preserved when the field is modulated. Some particles are swept into resonance with the oscillator earlier or later than others, and the displayed peak is broadened accordingly. The width of the peak illustrated is about 20 gauss, which means a difference of some 85,000 revolutions per second in the rate of precession of the slowest and fastest particles.

With an instrument of high resolution many substances show fine multiple peaks. This is due to the complex magnetic interaction between systems of particles and the consequent shielding of the biasing field. Many substances are not sensitive to an external magnetic field because the magnetism of their spinning particles cancels out. But those substances that do respond can be identified by the characteristic pattern which shows up on the 'scope. The resolution of the apparatus described here is not high enough for fine spectroscopic work. As indicated earlier, it is intended to serve as a simple demonstration of the magnetic-resonance effect.

Modifications to adapt the apparatus for limited applications would include the provision of larger pole faces on the magnetron magnet to provide a more uniform biasing field. In contrast with the 20-gauss peak-width displayed by the apparatus, the best instruments made today resolve to a few ten thousandths of a gauss; this means that irregularities in the biasing field must be kept below this figure. High resolution also requires precise and calibrated control of the intensity, frequency and amplitude of the biasing field. In this demonstration the high sweep-rate of 60 cycles per second is made possible by limiting the experiment to a test solution of ferric nitrate. Few substances are so responsive.

Incidentally, the magnetic-resonance spectrometer can also be used for measuring the strength of magnets. The magnet to be tested supplies the biasing field. It is modulated as described above, and the oscillator is adjusted to resonance. The strength of the unknown field in gauss is equal to the frequency of the oscillator when it is at resonance divided by 4,228.5.

The apparatus described has been in operation for a year and has been tested exhaustively for radiation hazard. The article might well have pointed out, however, that one invites trouble by remaining near particle accelerators when they are in operation, or even by staying in the same room with them during prolonged periods of operation. F. B. Lee, who designed the apparatus, does not share Bly's concern about the hazard of scattered electrons from the peephole. As a precaution, however, the peephole may be covered on the inside by a small piece of window glass which will plug the hole completely for electrons. As an alternative the neon tube can be cemented in the peephole and connected with its circuit through a pair of contacts (an arrangement that would permit the high-voltage terminal to be removed when desired). The lamp could then be observed at a distance with the machine in operation.

"First, the applicable laws and regulations make no distinctions in favor of U. S. citizens. Some other countries do discriminate against collection of fossils by non-citizens. To scientists it is a matter for pride that our country does not do so, and the fact should be known.

"Second, the collection of fossils on public lands is regulated by law, and the activities pursued by your correspondent and recommended to others are illegal. The pertinent Federal law is 34 Stat. L. 225, implemented by the 'Uniform Rules and Regulations' issued jointly by the secretaries of the Interior, Agriculture and Defense. Fossils in the public domain may be collected only under written permits issued by the department having jurisdiction over the land in question. Such permits can be issued only to reputable museums, universities, colleges, or other recognized scientific or educational institutions, or to their duly authorized agents. 'Conditions for granting a permit are very stringent and include the requirement that all fossils must be preserved in a public museum and be accessible to the public.

"The so-called 'Antiquities Act,' Stat. L. 225, and the accompanying regulations were intended primarily for the protection of archaeological sites and objects, but it has been ruled that they apply equally to all paleontological 'antiquities,' that is, to fossils. Many paleontologists, including me, think that the application to fossils is unduly restrictive and that the regulations could and should more realistically take account of differences between archaeological and paleontological collecting. Nevertheless these restrictions are now in effect, and your article urges your readers to commit illegal actions that would make them liable to arrest and to confiscation of their collections.

"The rationale of the Antiquities Act is that amateur or unregulated collectors of 'antiquities' can and frequently do cause irreparable harm-a consideration that rarely applies to amateurs in other sciences. That danger is particularly present in archaeology, and the most stringent regulation of collecting in that field is certainly justified. In paleontology the danger is slight in some circumstances but is also frequently serious. Professional paleontologists are therefore properly cautious about encouraging amateurs. It is true, as your article states, that amateurs 'can render substantial help to paleontologists.' When they do, we are grateful. More often, however, amateurs cause the permanent loss of priceless scientific data, and then we are definitely not grateful.

"Amateurs would be encouraged by most paleontologists if all of them would just follow a few simple suggestions. First, never collect a fossil unless (a) you know beyond any doubt that it has no research value, or (b) it is in obvious danger of destruction before a paleontologist can get to it, or (c) you can and do collect by professional standards, including the making of precise records of locality and stratigraphy. Second, if those conditions are not met, leave the fossil alone and notify a paleontologist. Third, if you do collect a fossil, submit it to a paleontologist, who will give you at least an approximate identification. If it has no unique research value, you may properly add it to your personal collection. If it does have such value, present it to a public institution-you will, incidentally, thus have a permanent memorial as its discoverer."

Suppliers and Organizations

The Society for Amateur Scientists
5600 Post Road, #114-341
East Greenwich, RI 02818
Phone: 1-401-823-7800

Internet: http://www.sas.org/