Note 8 — LECTURE I. ELECTRICAL INSTRUMENTS.

The Galvanometer as an aid to the Dosage of Electricity.—The dose of voltaic electricity is made up of two factors, (a) the strength of the current and (b), the time during which it is applied to the patient.|The Galvanometer as an aid to the Dosage of Electricity.| The strength of the current is directly dependent upon the number of cells employed, but, unfortunately, cells of dissimilar construction evolve currents of very unequal strength; while cells that have been freshly charged are more powerful than similar ones that have been partly exhausted by use; and, therefore, to speak of a current from “so many cells,” though, practically, a convenient method of dosage, fails to convey any exact idea of a measured and unvarying quantity. It is a comforting theory to electro-therapeutists that a galvanometer will enable them to administer their doses of electricity with as much exactitude as we daily prescribe so many grains, or so many minims of ordinary medicines; but, like some other theories which save us much trouble, when adopted as theories only, it fails us in practice (at least according to my experience), and chiefly so, because a galvanometer can be usefully employed only when it is included in the circuit of a continuous current, as, e.g., in aneurismal electro-puncture; and, I believe, I am within the mark in saying that electrizations, which even admit of its useful employment, are indicated in barely 5 per cent. of ordinary cases in electro-therapeutics; and that it is of no practical utility, where we most want aid, in measuring, not the current which leaves the battery terminals, but that which, after overcoming the very variable resistance of the human skin, really reaches the underlying muscular and nervous tissues, which, in 95 per cent. of our cases, we desire to influence, not by a constant, but by an interrupted Voltaic current; and the amount which really reaches these tissues depends largely upon the condition of the patient’s skin, and, I may also add, upon the kind and shape of the conductor, and its degree of moisture, &c.; and the operator will do well to graduate his dose of electricity by a consideration only of the three factors, number of cells, effect upon himself, and effect upon his patient, discarding entirely the use of any merely mechanical aids to graduation.

I am induced to speak thus strongly because men of scientific reputation have advocated the habitual use of the galvanometer, not alone by medical men trained to precision of observation, but by private patients as “enabling them to carry on the treatment at home with all the accuracy desirable!” The prospect of the ordinary patient provided with a battery, the use of which he is complicating by a galvanometer, is anything but reassuring to those physicians who not only prescribe electricity, but are themselves habituated in applying it—which, by the way, is a very different thing—and who have had frequent experience of the manner in which patients misunderstand, or fail in correctly carrying out, the most explicit directions. Electricity will be left in the hands of specialists, and necessarily do but a tithe of the good it is capable of affecting, until the mass of the profession can be induced to master the few preliminary details essential to its successful application, and I fear that the suggestions that have been made—suggestions which I believe to be entirely without foundation—that there exist practical difficulties to its dosage, will tend to postpone rather than to accelerate its more extended use.

Should any of you desire to use a galvanometer, that patented by Sprague, of Birmingham, is the one most adapted for use in medicine. Electricity is a force, and as with other forces it has its standard of measurement. In mechanics we know that the power sufficient to raise one pound to the height of one foot is the basis of measurement. Similarly in electricity the unit of measurement is the force which will raise one gramme to the height of one metre, and the standard multiple of this was called a “British Association Unit,” or shortly, a “B.A.” unit, and it is now called an “Ohm” when used to measure the resistance offered to the current, and a “Veber” when used to measure the strength of the current itself. The ordinary galvanometer is founded on the principle that a magnetic compass needle has a tendency to place itself at right angles to a current of electricity, and the degree to which the needle is deflected is a measure of the quantity of electricity, but the angle of deflection is not proportionate to the current strength, and it differs in different galvanometers; but in “Sprague’s Galvanometer” the dial is divided, not into degrees, but into divisions of thousandths of Vebers—divisions which were obtained by noting the deflections given by the needle with currents of known strength. I am indebted to Mr. Sprague for his courtesy in endeavouring to so modify his galvanometer as to render it available as a graduator of doses of interrupted Voltaic electricity, but although he has not succeeded in doing this, he has constructed for me an instrument which, supposing that a battery be partially exhausted, will indicate with precision the absolute strength of, say, twelve of its cells as compared with twelve newly-charged cells, and also the condition of each individual cell, points often of much practical convenience in an Hospital Electrical Room.