From the beginning days of radio

The following is from a paper published in the monthly magazine 'The Electrical Engineer'. It's author is one of the original investigators of radio phenomena and a true pioneer. He invented much of the apparatus used in radio in later years, the most significant was the crystal point contact diode detector. He was hounded into patenting it by his peers, though he never sought to profit by it. In 1895 he was publishing about his work, using 60GHz and demonstrating remote control in simple demonstrations. 

 THE ELECTRICAL ENGINEER, OCTOBER 2, 1896.

 ON A COMPLETE APPARATUS FOR THE STUDY OF THE PROPERTIES OF ELECTRIC WAVES*

BY JAGADIS CHUNDER BOSE, M.A. (CANTAB), D.SC (LOND.),

PROFESSOR OF PHYSICAL SCIENCE, PRESIDENCY COLLEGE,

CALCUTTA.

    * Paper read before the British Association at Liverpool,

The work of Herz and his eminent successors, both here and in the Continent, has opened out for study a new region of ethereal vibration, bridging over the gap that hither to existed between the comparatively slow electric vibrations and the quick oscillations which give rise to radiant heat. In the vast range of possible ether vibrations we recognise only a few octaves by our senses, the rest are beyond our perception. Many unexpected properties of these little-known ether waves are now being gradually discovered. Confining our attention to the electric waves, we find that there are many important problems which may perhaps be better attacked with these comparatively slow ether vibrations, among which maybe mentioned the determination of the indices of refraction of various substances which are opaque to visible light, but are transparent to the electric ray ; the relation between the dielectric constant and the refractive index when the rates of oscillation are made comparable in the two determinations—the variation of the index with the frequencies of vibration. Then there are the phenomena of double refraction, polarisation, and the magnetic rotation of the electric ray, the determination of the wave-length, and other problems of a similar nature. The fascination of the subject drew me to its study, though the investigations were rendered exceedingly difficult in India from want of facility for making the necessary instruments. I ultimately succeeded in constructing a few instruments, with which I was able to obtain the values of the indices of refraction of various substances for electric waves, the wave-length of electric radiation, and to demonstrate the phenomena of double refraction and polarisation of the electric rays. Before you is the simplified apparatus with which many of the properties of electromagnetic radiation may be studied. This is a duplicate made by Messrs. Elliott Bros, of the somewhat rudely-constructed apparatus which I brought from India. I also take this opportunity of thanking Mr. Bolton, F.R.A.S , of the Mathematical Instrument Department, Calcutta, for the divided circle in my apparatus. The following are the experiments which may be carried out with this apparatus :

A.—Verification of the laws of reflection :
(1) Plane mirrors ;
(2) curved mirrors.

B.—Phenomena of refraction:
(1) Prisms;
(2) total reflection ;
(3) opacity caused by multiple refraction and reflection ;
(4) determination of the indices of refraction.

C.— Selective absorption :
(1) Electrically-coloured media.

D.— Phenomena of interference.

E.—Double refraction and polarisation :
(1) Polarising gratings;
(2) polarising crystals ;
(3) double refraction produced by crystals ;
(4) double refraction produced by other substances ;
(5) double refraction produced by strain ;
(6) circular polarisation ;
(7) magnetic rotation ;
(8) electropolariscope and polarimeter.

In the list of experiments above mentioned, the determination of wave length by curved gratings has been carried out with a larger apparatus (see the current number of the Proceedings, Royal Society). Experiments with circular polarisation and magnetic rotation are still in progress. All the others have been repeated with the apparatus to be described below. The complete apparatus consists of—

(1) a radiating apparatus emitting electric waves of short length ;
(2) a receiver used as a detector of electric radiation ; and
(3) various accessories for the study of the different phenomena.

Radiating Apparatus.
Electric oscillation is produced by sparking between two beads of platinum and an interposed sphere of the same metal. The discharge ceases to be oscillatory when the ball is roughened, and a platinum ball resists, to a great extent, the disintegrating action of the sparks. Two jointed electrodes carry the two beads at their ends. The distance between the beads and the interposed sphere can thus be adjusted. This is a matter of importance, as the receiver does not properly respond if the spark length is too large. It is more convenient to use short electric waves, and this is obtained by making the radiating spheres very small. The shortest wave-length produced is about 6mm., and the corresponding number of oscillations is about 50,000 millions in a second. The frequency of vibration in this case will be seen to be about 13 octaves lower than that which produces visible radiation. The intensity of radiation in the above case is rather feeble, and I use in general electric waves of about 1cm. length.  The jointed electrodes carrying the beads are in connection with a small modified Ruhmkorff coil, actuated by a small storage cell. The usual vibrating interrupter is a source of trouble; the contact points get worn out, and the break becomes irregular.  I have, therefore, discarded it in favour of a simple interrupting key. The great objection to the continuous production

secondary sparks is the roughening of the surface of the radiating ball, by which the spark ceases to be oscillatory. It is very troublesome to be obliged to take out the radiator, in the middle of an experiment, for polishing. The flash of radiation produced by a single break is enough for an experiment, and it is a mere waste to have a series of useless oscillations. To economise space, I wind the condenser (a long strip of paraffined paper with tinfoil on opposite sides) round the secondary of the coil, appropriate connections being made with the interrupting key. The coil and a small storage cell are enclosed in a tinned iron box. Iron is used to screen the space outside from magnetic disturbance due to the making or breaking of primary circuit of the coil. A sudden magnetic variation disturbs the receiver. The iron box is placed inside a second box of thick brass or copper. These precautions are taken to prevent straying of electric radiation. Through a small opening behind the box the stud of the press key projects. In front of the box is the radiator tube, which may be square or cylindrical Inside this tube is mounted the radiating vibrator. A flash of electric radiation is produced by proper manipulation of the interrupting key. The radiating apparatus may thus be made very small and portable, and requires very little attention. After the storage cell is once charged, experiments may be carried on for days, a flash of radiation being produced at any time by merely manipulating the key.

Spiral Spring Receiver.
The receiving circuit consists of a spiral spring coherer, in series with a voltaic cell and a dead-beat galvanometer. The receiver is made by cutting a narrow groove in a rectangular piece of ebonite, and filling the groove with bits of coiled steel springs arranged side by side in a single layer. The spirals are prevented from falling by a glass slide in front. The spirals are placed between two pieces of brass, of which the upper one is sliding and the lower one fixed. These two pieces are in connection with two projecting metallic rods which serve as electrodes. An electric current enters along the breadth of the top spiral and leaves by the lowest spiral, having to traverse the intermediate spirals along the numerous points of contact. The resistance of the receiving circuit is thus almost entirely concentrated at the sensitive contact surface, there being little useless short-circuiting by the mass of the conducting layer. When electric radiation is absorbed by the sensitive surface, there is a sudden diminution of the resistance, and the galvanometer spot is violently deflected. By means of a very fine screw the upper sliding piece can be gently pushed in or out. In this way the spirals may be very gradually compressed, and the resistance of the receiver diminished. The galvanometer spot can thus be easily brought to any convenient position of the scale. When electric radiation falls on the sensitive surface the spot is deflected. By a slight unscrewing the resistance is increased, and the spot made to return to its old position. The receiver is thus re-sensitised for the next experiment. The sensitiveness of the receiver may be increased by a proper adjustment of the E.M.F. acting on the receiving circuit. The receiver at each particular adjustment responds best to a definite range of vibration lying within about an octave. The same receiver could, however, be made to respond to a different range by an appropriate change of the E.M.F. ; very careful adjustment of this is necessary to make the receiver respond at its best to a particular range of electric vibration. For simple experi- ments the adjustment of the receiver is not difficult, but for delicate experiments careful manipulation is necessary. The proper adjustment of the E.M.F. is effected by taking a derived current from a circular potentiometer slide, fixed at the base of the galvanometer. A simpler way is to take a U tube, the two limbs being respectively filled with copper sulphate solution and dilute sulphuric acid. Mixture of the two solutions is prevented by an interposed plug of asbestos. A rod of copper and a rod of zinc are plunged in the two electrolytes, the whole forming a modified Daniell cell. The cell is shunted by a suitable resistance, the receiving circuit being connected to the ends of the shunt. The current flowing through the shunt, and therefore the derived E M.F. from its ends, is varied by plunging the rods more or less in the solutions. The leading wires from the ends of the receiver are enclosed in layers of tinfoil ; the galvanometer and cell have a metallic cover with a slit for the passage of reflected spot of light. The receiving circuit is thus shielded from the disturbing action due to stray radiations. The receiver is provided with a collecting funnel. This prevents lateral waves from acting on the receiver. The funnel has two hinged side-doors by which its area—and, therefore, the amount of radiation collected— may be varied. When angular deviation is to be measured, the doors are made parallel and perpendicular to the layer of spirals. The aperture is reduced, and the receiver then only responds when the funnel points to the direction of the deviated ray. In polarisation experiments it is necessary to adjust the receiver carrying the analyser in a crossed position. This is done by a tangent screw, the rotation of the analyser being measured by means of an index and a graduated vertical disc.

Arrangement of the Apparatus.
The radiating apparatus and the receiver are mounted on stands sliding in an optical bench. Experiments are carried out with divergent or parallel beam of electric radiation. To obtain parallel  beam a cylindrical lens of sulphur or ebonite is mounted in a square tube. This lens tube fits on the radiator tube, and  stopped by a guide when the oscillatory spark is at the principal focal line of the lens. The radiator tube is further provided with a series of diaphragms by which the amount of radiation may be varied. For experiments requiring angular measurement, a spectrometer circle is mounted on one of the sliding stands. The spectrometer carries a circular platform on which the various reflectors, refractors, etc., are placed. The platform carries an index, and can rotate independently of the circle on which it mounted.  The receiver is carried on a radial arm (provided with an index) and points to the centre of the circle. An observing telescope may also be used with an objective made of ebonite with a linear receiver at the focal plane. But an ordinary receiver provided with a funnel is all that is necessary for ordinary experiments.

Laws of Reflection.

Plane Mirrors.
A parallel beam is used. The spectrometer circle is adjusted with the zero division opposite to the radiator. The platform index is turned to zero, and a plane reflector placed on a previously marked diameter at right angles to the index. The receiver isplaced, say, at 60deg. The platform carrying the mirror is slowly rotated (electric radiation being at the same time produced by tapping the key) till the receiver suddenly responds. It would now be found that the platform index points to 30deg., midway between the radiator and the receiver.

Curved Mirror
A cylindrical metallic mirror, with a radius of 26cm., is placed on the platform, with its principal axis coinciding with the platform index. When the radiator is placed at a distance of 26cm. from the mirror the source of radiation would be at the centre of the cylinder. The reflected image will now be formed at an equal distance. The receiver mounted on the radial arm (at a distance of 26cm. from the centre) is placed at a given angle; the platform is rotated till the receiver responds. The index would now be found to bisect the angle included between the radiator and the receiver.

Refraction.

Deviation of the Electric Ray by a Prism.
An isosceles right angled prism is made of sulphur or ebonite. Parallel beam is used. For showing deviation by refraction one of the acute angles is interposed on the path of the beam.

Total Reflection
An interesting experiment on total reflection is shown in the following way : The receiver is placed opposite to the radiator, and the prism interposed with one of its equal faces at right angles to the direction of tho ray. The receiver will remain unaffected. The critical angle of ebonite being consider ably less than 46deg., the rays undergo total reflection. On turning through JKMeg.* the receiver responds to the totally reflected ray.

Opacity due to Multiple Refraction and Reflection.
An experiment analogous to the opacity of powdered glass to light U shown by filling a long trough with irregular-shaped pieces of pitch and interposing it between the radiator and the receiver. The electric ray is unable to pass through the heterogeneous media owing to the multiplicity of refractions and irregular reflections, and the receiver remains unaffected ; but on restoring partial homogeneity by pouring in kerosine,which has about the same refractive power as that of pitch, the radiation is easily transmitted.

Determination of the Indices of Refraction.
Two semi-cylinders of the given substance separated by an air film is placed on the platform. When the radiator is placed at the principal focus of one of the semi-cylinders the rays emerge parallel into the air film, and are then focused on the receiver by the second semi-cylinder. A metallic plate with a narrow rectangular opening is interposed between the semi-cylinders to serve as a diaphragm, and cut off all but the central rays. As the platform is rotated the incident angle on tho plane surface separating the two media is gradually increased till the rays undergo total reflection When this is the case the receiver ceases to respond. The index reading is now taken, and the cylinders rotated in an opposite direction till total reflection takes place a second time. The difference of readings as given by the index in the two positions is evidently equal to twice the critical angle. Hence the value of the index can easily be deduced. A preliminary experiment gives the approximate value of the index, from which the focal distance of the semi-cylinder is roughly calculated. The spark gap of the radiator is placed at this focus, and the experiment repeated. In this way I have determined the indices of refraction of several solids for the electric ray (Proceedings of the Royal Society, vol. lix ). The index of refraction of commercial sulphur is — 1.73,
that of a specimen of pitch = 1.48.
Indices for Liquids.—A cylindrical trough is filled up with the given liquid ; two thin parallel glass plates enclosing an air film are vertically placed so as to divide the liquid cylinder into two halves. The readings for total reflection are taken as in the last case. The index for coal tar I found to be 1.32.

Selective Absorption.
A substance is said to be coloured when it allows light of one kind to pass through but absorbs light of a different kind  If we take into account the entire range of radiation there is hardly a substance which is not in this sense coloured. In the spectrum of radiation transmitted through glass for example, two broad absorption bands would be observed, one in the ultra-violet and the other in the infra-red, the electric and the visible rays being not absorbed to any great extent. A brick or block of pitch would absorb light, but would transmit the electric ray. On the other hand, a stratum of water, though transparent to light, would absorb the electric ray. These substances exhibit selective absorption, and are therefore coloured. If we only take into account the electric radiation it would no doubt be found that radiations having different wave-lengths are unequally absorbed by different substances.

Phenomena of Interference.
Determination of the Wave-Lengths by Diffraction Grating.
In a paper read before the Royal Society in June last I have given an account of a method of obtaining pure spectra of electric radiation by means of a curved grating. The experiment was carried out with a large apparatus. The spectrum obtained was well defined, and appeared to be linear, and not continuous. I had not time to adapt the experiment to this small apparatus, but I think it would not be difficult to do so.

Double Refraction and Polarisation.
The spectrometer circle is removed, and an ordinary stand for mounting the receiver substituted. By fitting the lens tube, the electric beam is made parallel. At the end of the lens tube there is a slot in which is dropped the wire grating polariser. A crystal holder provided with three sliding jaws is fitted on to the lens tube, and is capable of rotation round an axis parallel to the direction of the electric ray. The receiver carrying the analyser is also capable of rotation round a horizontal axis by means of a tangent screw. The angular rotation is measured by means of an index fixed to the analyser and a graduated vertical disc. The gratings are made by winding fine copper wire parallel round square frames. Other forms of polariser will be described later on. The spark gap is placed vertical, and the polariser is adjusted with wires horizontal. The emergent beam is now completely polarised, the vibration taking place in a vertical plane passing through the axis. The analyser fitted on to the receiver may be placed in two positions:
(1) parallel position—when both the gratings are horizontal ;
(2) crossed position—when the polarising grating is horizontal and the analysing grating vertical. In the first position, the radiation being transmitted through both the gratings, falls on the sensitive surface, and the galvanometer responds. The field is then said to be bright. In the second position the radiation is extinguished by the crossed gratings, the galvanometer remains unaffected, and the field is said to be dark. But on interposing certain crystals with their principal planes inclined at 45deg. To the horizon the field is partially restored, and the galvanometer spot sweeps across the scale. This is the so-called depolarisation action of double-refracting substances.

Experiments with Wire Gratings.—A wire grating at 45deg. interposed between the crossed analyser and polariser partially restores the field, but ordinary wire gauze does not transmit any radiation, the action of one set of wires being neutralised by that of the other set at right angles. (For a detailed account of experiments on the polarisation of the electric ray, I would refer to my paper read before the Asiatic Society of Bengal, May, 1895)

Double Refraction Produced by Crystals.—The crystals to be examined are mounted on the holder, and properly inclined. Double refraction is shown by all crystals belonging to the rhombic, rhombohedral, triclinic, and monoclinic systems. The effects exhibited by the following are very marked :
(1) Sorpcntine : This crystal, which appears fibrous, transmits the ordinary and the extraordinary rays with unequal intensity. A fairly thick piece would absorb vibrations parallel to the fibres, and transmit vibrations perpendicular to the fibres. Ordinary radiation, after transmission through a thick piece of serpentine, would be plane polarised, the vibration taking place perpendicular to the fibres.
(2) Nemalite : This crystal exhibits this effect in a still more marked decree.
(3) Tourmaline also produces the depolarisation effect. The difference in absorption of the ordinary and the extraordinary rays is, however, not so great as in the case of light.
(4) Beryl is also very good crystal for exhibiting the depolarisation effect.

Polarisation Produced by other Substances.
I found many other natural substances producing polarisation, the most interesting being vegetable fibres. Common jute (Corchorus capxularis) exhibits the property in a very marked degree. I cut fibres of this material about 3cm. in length, and built with it a cell with all the fibres parallel. I subjected this cell to a strong pressure under a press. I thus obtained a compact cell 3cm. by 3cm. in area and about 5c. in. in thickness. This was mounted in a metallic case, with two openings about 2cm. by 2cm. on opposite sides for the passage of the radiation. This cell was found to quench vibrations parallel to the fibres, and transmit vibrations perpendicular to the fibres. Jute cells could thus be made to serve as polarisers or analysers.

Effects Due to Strain
Could be exhibited by stratified rocks, the plane of stratification being inclined at 45deg. to the horizon. Effects similar to that produced by unannealed glass can be imitated by a block of unequally chilled paraffin. The polarisation apparatus above described may also be used as a polarimeter, the rotation of the analyser being measured by the graduated disc.