DYNAMITE 



1893 



DYNAMO 



ured by its effect on a given mass, and two 

 systems or units are used. These are the 

 joot-pound, or poundal (which see) of the 

 English system, and the dyne (which see) of 

 the metric system. In English-speaking coun- 

 tries the metric system is generally used in 

 physical laboratories, and the English system 

 elsewhere. See PHYSICS. 



The resistance which a body offers to a force 

 is directly in proportion to its mass. That is, 

 a body weighing four pounds will offer four 

 times the resistance to a given force that a 

 body weighing one pound will offer. This may 

 be expressed in the following law: 



The velocity produced in a body free to move 

 without resistance in any unit of time will be 

 directly proportional to the intensity or amount 

 of the impressed force and inversely propor- 

 tional to the mass of the body. 



The science of dynamics is founded almost 

 entirely on the following three principles, 

 known as Newton's Laws of Motion: 



(1) A body at rest remains at rest, and a 

 body in motion moves with a uniform velocity 

 in a straight line, unless acted upon by some 

 external force. 



( 2 ) A given force will produce the same effect, 

 whether acting upon a body in motion or at rest, 

 whether the body is acted upon by that force 

 alone or by others at the same time. 



(3) The action of every force is accompanied 

 by an equal reaction in the opposite direction. 



DYNAMITE, di'namite, a word derived 

 from the Greek dynamis, meaning power, and 

 applied to an explosive substance invented by 

 A. Nobel in 1866. It consists of a mixture 

 of nitroglycerine and some absorbent material 

 such as infusorial earth, wood pulp, guncotton, 

 or mixtures of such materials with nitrates, such 

 as saltpeter. Nobel found that by soaking gun- 

 powder in nitroglycerine an explosive of tre- 

 mendous power was obtained. Since 1866 the 

 processes for the preparation of dynamite have 

 undergone many modifications. All experi- 

 ments have been directed toward the produc- 

 tion of a dynamite suitable for blasting pur- 

 poses, combining power with comparative 

 safety in handling. 



The most powerful dynamite is now made 

 in the form of gelatin consisting of cellulose 

 nitrate dissolved in nitroglycerine. This is 

 mixed with wood pulp and a little potassium 

 nitrate. The result is a jelly which is easily 

 handled and is suitable for blasting purposes 

 in either dry or wet localities. For commercial 

 purposes dynamite is put up in sticks, or cart- 

 ridges, eight inches long and from one to three 

 inches thick. The percentage of nitroglycerine 



varies according to the desired strength. The 

 average dynamite for blasting contains forty 

 per cent of nitroglycerine, but sometimes this 

 is increased to seventy per cent. The dynamite 

 is exploded by means of a fuse and a detonating 

 cap of fulminate of mercury. The explosive 

 force of dynamite is thirteen times as great as 

 that of ordinary gunpowder. It ranks so far 

 below more recently-discovered explosives, 

 however, that except in combination it was 

 little used in the War of the Nations, which 

 began in 1914. See EXPLOSIVES. J.F.S. 



DYNAMO, a machine that generates elec- 

 tricity for light, heat, power and other com- 

 mercial purposes. The name comes from a 

 Greek word meaning power. The three essen- 

 tial parts of a dynamo are the field magnet, 

 the armature and the commutator. To under- 

 stand the construction of a dynamo and the 

 principle upon which it works, one must have 

 some knowledge of electricity, magnetism and 

 mechanics. A brief description to the non- 

 technical mind can at best give an account 

 of only the most important parts and their 

 operation. 



Principle of the Dynamo. The principle 

 upon which the dynamo is built is that mag- 

 netism generates electricity, and electricity gen- 

 erates magnetism. If a steel bar magnet is 

 thrust into the interior of a coil of wire, a 

 c 



d 

 FIG. 1 



current of electricity will be generated in the 

 wire. The pressure of this current is greater 

 at one end of the wire and the current will 

 flow continuously in the direction of the least 

 resistance so long as conditions remain un- 

 changed ; but if the magnet is turned half way 

 round so as to reverse its poles, the current will 

 flow in the opposite direction. In Fig. 1, n and 

 s represent the poles of a niagnet. The space 

 between them is a field of magnetic force. The 

 wire c d is placed at right angles to the lines 

 of this force. If the wire is moved downward 

 the current flows from d to c; if it is moved up- 

 ward the current is from c to d. That is, every 

 time the position of the wire changes in refer- 

 ence to the magnet, the direction of the current 

 changes. If in place of the single wire we use 

 a rectangular wire frame mounted on an axis 



