In the book, Tesla — Man Out of Time, the author wrote about a "death ray" that Tesla had reportedly invented.  Could you tell me what exactly the Death Ray was and how it worked?

Also called the "Peace Ray" and "teleforce," the account refers to Tesla's macroscopic particle beam projector, first mentioned publicly in "Invents Peace Ray," New York Sun, July 10, 1934 [Collected Articles of Nikola Tesla, Volume 3].  The device, which was based upon a large Van de Graaff generator of unique design and a special type of open ended vacuum tube, comprised a system for the acceleration of very small charged metallic particles to prodigiously high velocity, according to Tesla about 48 times the speed of sound.  The particles were projected out of the tube by means of electro-static repulsion.  As can be seen from Tesla's words written in a letter to J.P. Morgan, Jr. on November 29, 1934 it was intended for use in national defense.

   "I have made recent discoveries of inestimable value which are referred in the marked passage of the clipping enclosed. . . [possibly "Dr. Tesla Visions the End of Aircraft In War," Syracuse Herald, October 21, 1934]  The flying machine has completely demoralized the world, so much that in some cities, as London and Paris, people are in mortal fear from aerial bombing.  The new means I have perfected afford absolute protection against this and other forms of attack."

The tube was designed to project a single row of highly charged particles.  According to Tesla there would be no dispersion whatever, even at great distance.  Since the cross section of the carriers might be reduced to almost microscopic dimensions, an immense concentration of energy, practically irrespective of distance, could be attained.

From Tesla's words, written in a letter to J. P. Morgan, Jr., on November 29, 1934, it can be seen the device was intended for use in national defense.

—I have made recent discoveries of inestimable value. . . . The flying machine has completely demoralized the world, so much that in some cities, as London and Paris, people are in mortal fear from aerial bombing. The new means I have perfected afford absolute protection against this and other forms of attack. . . . These new discoveries, which I have carried out experimentally on a limited scale, have created a profound impression.  One of the most pressing problems seems to be the protection of London and I am writing to some influential friends in England hoping that my plan will be adopted without delay.  The Russians are very anxious to render their borders safe against Japanese invasion and I have made them a proposal which is being seriously considered.— 

According to Tesla production of the particle beam is dependent upon the following four inventions:

1) A method and apparatus for producing rays and other manifestations of energy in free air, eliminating the high vacuum necessary at present for the production of such rays and beams.  This is accomplished with a novel form of high vacuum tube, one end of which is open to the atmosphere.  The projectiles are accelerated in a vacuum and then conducted into the atmosphere through a valvular conduit. 

2) A method and process for producing very great electrical force in the range of 60,000,000 volts to propel the particles to their objective.  Tesla specified that this could be done with a large electrostatic generator on a new principle and of very great power, in many respects similar to a Van de Graaff generator.  In place of a charge-carrying belt it employs a circulating stream of desiccated air that is propelled through a hermetically sealed ductwork by a Tesla disc blower.  A Wardenclyffe type apparatus could also be used for this purpose.

3) A method for amplifying this process in the second invention.  The exterior of the high potential terminal is equipped with numerous bulbs of some insulating material each containing, —an electrode of thin metal sheet suitably rounded— and —exhausted to the highest vacuum obtainable.— 

4) A new method for producing a tremendous electrical repelling force in the form of —provisions for imparting to a minute particle an extremely high charge.—  It appears this refers to the internal conducting component (the socket and central extension) at the base of the projector or gun element of the system   While the specific details about this aspect of the design are not readily apparent, it seems that strict attention to the fulfillment of requirements 1, 2 and 3 is critical to success.  In Tesla—s words, —by the application of my discoveries it is possible to increase the force of repulsion more than a million times and what was heretofore impossible is rendered easy of accomplishment.—[1, 2]  

     —When put in operation Dr. Tesla said this latest invention of his would make war impossible.  This death-beam, he asserted, would surround each country like an invisible Chinese wall, only a million times more impenetrable. It would make every nation impregnable against attack by airplanes or by large invading armies. 
     —But while it will make every nation safe against any attack by a would-be invader, Dr. Tesla added, the death-beam by its nature could not be employed similarly as a weapon for offense.  For this death-beam, he explained, could be generated only from large, stationary and immovable power plants, stationed in the manner of old-time forts at various strategic distances from each country's border.  They could not be moved for the purposes of attack. 
     —An exception, however, he added, must be made in the case of battleships, which, he said, would be able to equip themselves with smaller plants for generating the death-beam, with enough power to destroy any airplane approaching for attack from the air.— [4] 

     —The beam of force itself, as Dr. Tesla described it, is a concentrated current—it need be no thicker than a pencil—of microscopic particles moving at several hundred times the speed of artillery projectiles.  The machine into which Dr. Tesla combines his four devices is, in reality, a sort of an electrical gun. 
     —As Dr. Tesla explained it, the tremendous speed of the particles will give them their destruction-dealing qualities.  All but the thickest armored surfaces confronting them would be melted through in an instant by the heat generated in the concussion." [5] 

     —My invention requires a large plant, but once it is established it will be possible to destroy anything, men or machines, approaching within a radius of 200 miles.  It will, so to speak, provide a wall of power offering an insuperable obstacle against any effective aggression.
     "I want to state explicitly that this invention of mine does not contemplate the use of any so-called "death rays."  Rays are not applicable because they cannot be produced in requisite quantities and diminish rapidly in intensity with distance.  All the energy of New York City (approximately two million horsepower) transformed into rays and projected twenty miles, could not kill a human being, because, according to a well known law of physics, it would disperse to such an extent as to be ineffectual."
     —My apparatus projects particles which may be relatively large or of microscopic dimensions, enabling us to convey to a small area at a great distance trillions of times more energy than is possible with rays of any kind.  Many thousands of horsepower can thus be transmitted by a stream thinner than a hair, so that nothing can resist.  This wonderful feature will make it possible, among other things, to achieve undreamed-of results in television, for there will be almost no limit to the intensity of illumination, the size of the picture, or distance of projection.— [6] 

In reference to, —his atom-smashing tube,— . . . it is not an experiment.  —I have built, demonstrated and used it.  Only a little time will pass before I can give it to the world.— [7] 

—As though I am poor with words.  I still didn't explain it enough what would be necessary to increase up to twelve stations: eight of them, each of the same construction like at Wardenclyffe and only 20 meters high--a ball five meters in diameter--the station would be using diesel oil for energy with mechanical action--my air turbines, steam powered, electrically or other manners of transforming into alternating electrical current with sixty million volts pressure without danger. . . . In my attempts with an effective 20 million volts . . . [particles] penetrated two meters in depth and [did] terrible damage . . .— [8] 

In 1934 Tesla spoke with an S. M. Kintner of the Westinghouse Electric & Manufacturing Company regarding further development of the particle-beam device.  There are two letters dated April 5, 1934 and April 7, 1934 that refer to these discussions.

A fairly detailed description of Tesla's particle-beam projector is presented in the book Nikola Tesla's Teleforce & Telegeodynamics Proposals.  A number of teleforce-related newspaper articles appear in the book Collected Articles of Nikola Tesla, Volume 3.

1. Tesla, Nikola, —New Art of Projecting Concentrated Non-Dispersive Energy Through Natural Media.— 

2. Nikola Tesla's Teleforce & Telegeodynamics Proposals, Leland Anderson and Gary Peterson, editors, 21st Century Books, 1998. 

3. "'Death Ray' for Planes", New York Times, September 22, 1940.  —Nikola Tesla, one of the truly great inventors who celebrated his eighty-fourth birthday on July 10, tells the writer that he stands ready to divulge to the United States government the secret of his 'teleforce,' of which he said, 'airplane motors would be melted at a distance of 250 miles, so that an invisible 'Chinese Wall of Defense' would be built around the country against any enemy attack by an enemy air force, no matter how large.'— 

4. "Tesla, at 78, Bares New 'Death Beam'", New York Times, July 11, 1934. Retrieved on 2007-07-21. —Invention Powerful Enough to Destroy 10,000 Planes at 250 Miles Away, He Asserts Defensive Weapon Only Scientist, In Interview, Tells of Apparatus That He Says Will Kill Without Trace.  Nikola Tesla, father of modern methods of generation and distribution of electrical energy, who was 78 years old yesterday, announced a new invention, or inventions, which he said, he considered the most important of the 700 made by him so far.  He has perfected a method and apparatus, Dr. Tesla said yesterday in an interview at the Hotel New Yorker, which will send concentrated beams of particles through the free air, of such tremendous energy that they will bring down a fleet of 10,000 enemy airplanes at a distance of 250 miles from a defending nation's border and will cause armies of millions to drop dead in their tracks.— 

5. "Beam to Kill Army at 200 Miles, Tesla's Claim On 78th Birthday", New York Herald Tribune, July 11, 1934.  —Dr. Nikola Tesla, inventor of polyphase electric current, pioneer in high frequency transmission, predecessor of Marconi with the wireless, celebrated his seventy-eighth birthday yesterday by announcing his invention of a beam of force somewhat similar to the death ray of scientific romance. It is capable, he believes, of destroying an army 200 miles away; it can bring down an airplane like a duck on the wing, and it can penetrate all but the most enormous thicknesses of armor plate. Since it must be generated at stationary power plants by machines which involve four electrical devices of the most revolutionary sort, Dr. Tesla considers it almost wholly a defensive weapon. In peace times, he says, the beam will also be used to transmit immense voltages of power over distances limited only by the curvature of the earth.— 

6. "A Machine to End War", Liberty (magazine), February 1937. —Tesla. 'It seems,' he says, 'that I have always been ahead of my time.' Editor's Note: Nikola Tesla, now in his seventy-eighth year, has been called the father of radio, television, power transmission, the induction motor, and the robot, and the discoverer of the cosmic ray.  Recently he has announced a heretofore unknown source of energy present everywhere in unlimited amounts, and he is now working upon a device which he believes will make war impracticable.  Tesla and Edison have often been represented as rivals.  They were rivals, to a certain extent, in the battle between the alternating and direct current in which Tesla championed the former.  He won; the great power plants at Niagara Falls and elsewhere are founded on the Tesla system.  Otherwise the two men were merely opposites.  Edison had a genius for practical inventions immediately applicable.  Tesla, whose inventions were far ahead of the time, aroused antagonisms which delayed the fruition of his ideas for years.  However, great physicists like Kelvin and Crookes spoke of his inventions as marvelous.  'Tesla,' said Professor A. E. Kennelly of Harvard University when the Edison medal was presented to the inventor, 'set wheels going round all over the world. . . . What he showed was a revelation to science and art unto ail time.'— 

7. "Sending of Messages to Planets Predicted by Dr. Tesla On Birthday", New York Times, July 11, 1937.  —Inventor, 81, Talks of Key to Interstellar Transmission and Tube to Produce Radium Copiously and Cheaply.  Decorated by Yugoslavia and Czechoslovakia.  Reports of discoveries by which it will be possible to communicate with the planets and to produce radium in unlimited quantity for $1 a pound were announced by Dr. Nikola Tesla yesterday at a luncheon on his eighty-first birthday at which he wee honored with high orders from the Yugoslav and Czechoslovak Governments.  Dr. Tesla, whose discoveries in electrical science have won for him recognition as the father of modern methods of generating and distributing electrical energy, asserted his "absolute" belief that he would win the Pierre Guzman prize of the Institute of France for his discovery relating to the interstellar transmission of energy.  Following his annual custom, Dr. Tesla played host to a group of newspaper men at his birthday luncheon at the Hotel New Yorker and issued the announcement of his discoveries of the last year.  No apparatus or sketches were shown, but Dr. Tesla said in announcing perfection of the principle of a new tube, which he said would make it possible to smash the atom and produce cheap radium, that he would be able to give a demonstration in 'only a little time.'— 

8. Correspondence from Nikola Tesla to Sava Kosanovic, New York, N.Y., March 4, 1941