circuit is said to be "open" when the wires are separated, "closed" when they are in contact.

The property of creating difference of potential, upon which the electrical current depends, is known as electro-motive force.

Electric force moves matter. Electro-motive force is an imaginary force moving an imaginary fluid electricity; its amount depends entirely upon the nature of a cell, and not at all upon its size.

To secure accuracy of admeasurement "electrical units" are employed; the more important of these are as follows:

"The use of the term Weber is restricted to the unit of quantity; the Ampere denotes the same quantity, but includes the twin factor implied in the word current."

"The Volt is a little less than the electro-motive force of a freshly-charged Daniell's cell."

All the parts in a galvanic circuit offering more or less resistance to the passage of a current, obviously the current strength (C), will be equal to the electro-motive force (E) divided by the resistance (R), or C = (Ohm's law).

The resistance offered by the wires is inversely proportional to their sectional area and directly to their length; it also varies with the specific conducting power of the metal of which they are composed.

The resistance of the human body, practically that of the epidermis, is very great; it may be diminished

by increase in the size of the electrodes, of the amount of pressure made by them, and of the duration of their application; also by increase in the moisture and vascularity of the skin.

By electric density is meant the proportion existing between the sectional area of a conductor and the quantity of electricity conveyed by it; the greater the sectional area, the quantity remaining the same, the less the density.

Arrangement of Elements.-In the simple circuit or arrangement in surface all the zinc plates are connected, forming the negative pole, all the carbon plates forming the positive pole. See Fig. 3.

When the carbon of the first cell is connected with the zinc of the second, and so on throughout the series, the first zinc forming the negative and the

last carbon the positive pole, we have a compound circuit or arrangement in series. See Fig. 4.

The cells may also be variously grouped. The arrangement in any particular case will depend upon the amount of resistance to be overcome, and upon the object to be attained. As a general rule, the grouping should be such as to make the internal resistance as nearly as possible equal to the external. For ordinary galvanic application to the human body, whose resistance is very great, the cells are connected in series; whereas, for electrolytic and heating purposes, the arrangement in surface is adopted.

When the current is passed through a portion of the body, a flow of the interstitial fluids occurs from the anode to the kathode; this is known as the cataphoric action of galvanism.

Luminous and heating effects may be obtained by causing a strong current to pass through a platinum wire.

Chemical Effects.-Electrolysis is the decomposition of a body,-oxygen and acids collecting at the anode; hydrogen, alkalies, and bases at the kathode.

(b) Faradism, or Induced Electricity. "The faradic current is generated by the inductive influence of galvanism or magnetism, in presence of variation in intensity of this influence." An electrical current is developed in a closed coil of wire when a magnet is brought near, and another, but in the opposite direction when it is withdrawn. Upon this principle depends the construction of the old magneto-electrical apparatus, which the recent discovery of a commutator, by which the currents may be collected and sent in the same direction, will probably again bring into use. A galvanic current induces a current in a neighboring wire, at the moment of closure in the opposite direction, at the moment of opening in the same direction.

When the current traverses an insulated helix within which is placed a soft iron bar, the bar becomes a magnet on closure of the circuit, to be instantly demagnetized when it is broken. These are the principles involved in the construction of the electro-magnetic, the common faradic battery. Their application can be clearly understood from

Fig. 5.