That it is used not infrequently merely to act upon the patient's mind when other therapeutic resources have been exhausted, cannot be denied ; but such use, whether legitimate or not, does not undervalue its status when scientifically employed for well-established indications. To accomplish good results, it must be used according to fixed laws which are simple and easily understood; and with increased knowledge comes a higher opinion of the value of the remedy, and the disbeliever often changes to an enthusiast.

With a daily enlarging knowledge and a constantly widening field of applicability, no physician can expect to compete with his brethren in therapeutic skill, who fails to secure at least an average acquaintance with the science.

The undergraduate also will find that he cannot afford to be without an understanding of the subject; for he will become aware that the building of his practical learning upon the largely-theoretical foundation laid during his college years will be very seriously handicapped by the omission to give this subject a fair share of his time. He cannot be expected to read exhaustive works, but a very good

idea of the rudiments is to be expected; and since an understanding of medical and surgical Electricity would be difficult, if not impossible, without a knowledge of at least the elements of physical science, so much of the latter as is necessary for the proper assimilation of the text has been inserted.

The use of technical terms employed in expressing the units of force, quantity, etc., has become so common, that an acquaintance with their significance may be considered indispensable; hence their use in the following pages is an advantage, and, being explained in a simple manner, does not give rise to any difficulty in comprehensibility.

CHARLES H. MAY, M. D.

202 East 58th Street,

New York, Jan. 3d, 1887.

CHAPTER I.

PRINCIPLES OF ELECTRO-PHYSICS.

I. MAGNETISM.-Magnetism is the property certain bodies have of attracting iron; substances possessing this power being known as magnets.

The natural magnet is the magnetic oxide of iron or lodestone, a not uncommon ore.

The artificial magnet is made by rubbing a piece of soft iron or steel with a natural magnet, or by the action of galvanism, the piece of metal being placed within a helix of insulated wire, through which the current is made to pass.

Soft iron ntagnets soon lose their power and are hence known as temporary magnets, but steel magnets are permanent.

Magnetic substances are such metals as iron, steel, nickel, etc., which are attracted by and are capable of being made magnets. Diamagnetic substances are

such as are repelled by a magnet.

If a magnet be suspended so as to move freely,

it will be found that one end always points to the north, while the other points to the south; hence the terms north and south pole of the magnet. Bring a second magnet into the neighborhood of the first, and we see that “like poles repel, unlike poles attract."

Armatures are pieces of soft iron used to connect the poles of a magnet; the iron becomes temporarily magnetized, and reacting on the magnet prevents any loss of its power.

Magnetic curves are the imaginary concentric lines radiating from each pole of the magnet, through which experiment has shown the magnetic force to be exerted; the space embraced by these curves is known as the magnetic field.

Magnets are of various shapes; that of a bar, or of a horseshoe is usually employed.

A magnetic battery consists of a number of magnets bound together with their like poles in the same direction.

II. ELECTRICITY.-"Electricity is a powerful physical agent, the existence of which is made known chiefly by attraction and repulsion, but also by luminous and heating effects, and various other

phenomena. Its action is excited by friction, pressure, chemical operations, heat and magnetism."

1. Static or Frictional Electricity is that form which is excited by friction; it is called static in contradistinction to dynamic or current electricity, because it is not in motion, but is restrained in a state of high tension.

It is supposed that there are two kinds of electricity pervading all bodies, positive or vitreous, and negative or resinous; in the unelectrified condition these fluids just neutralize each other, but may be separated by chemical action, friction, etc.

If a glass rod be rubbed with a piece of silk it will attract a pith ball, charge it with positive electricity and at once repel it; if the positively charged pith ball be now brought near a negatively electrified substance, the ball will be attracted, negatively charged by conduction, and again repelled; hence the law, "unlike electricities attract, like electricities repel."

Conductors are substances which offer very little resistance to the passage of a current of electricity through them; those bodies offering considerable resistance are known as non-conductors or insulators.