(A) Current Measurers

(1) Siemens's Electro - dynamometer consists of a fixed coil, and of a movable coil suspended by a thread and a spiral spring, the normal position of the latter being at right angles to that of the former.. When a current is sent through the 2 coils in series, the movable coil is deflected; this deflection is counteracted by torsion of the spiral spring, the amount of such torsion (indicated by a pointer on an index - dial) being proportioned to the square of the current strength.

(2) Obach's Tangent Galvanometer has a movable ring round a horizontal axis. If the ring is vertical, the needle tends to turn on a vertical axis, as in the ordinary tangent galvanometer (the opposing force being the horizontal component of the earth's magnetism); if, however, the ring is placed horizontally, the needle turns round a horizontal axis, or dips (the opposing force being the vertical component of the earth's magnetism). The effect of the intensity of the current upon the needle may therefore be varied with the inclination of the ring. With the ring in any fixed position, the current strength is proportional to the tangent of the deflection (as in an ordinary tangent galvanometer).

(3) Cardew's Low - resistance Galvanometer is based on the principle of finding the intensity of an unknown current by balancing it against one of known strength, and thus ascertaining the relation between the two. It con-sists of 2 coils, a thick wire one (for the unknown current), and a thin wire one (for the standard one); both are wound on the same bobbin, but are insulated from each other. A magnet, with needle - pointer, is pivoted in the centre of the bobbin, and its deflections are counteracted by the insertion of known resistance.

(4) Ayrton And Perry's Am - Meter

This instrument is made in several forms. In its original shape, it consists of a permanent magnet of horse - shoe pattern, with a needle - armature fixed between its pole - pieces. These are made of such a shape as to ensure that the deflections of the needle shall be directly proportional to the intensity of the current causing them; hence a knowledge only of the constant of each instrument is required when using it. Furthermore, the coil of wire through which the current passes, is in some instruments divided up into 10 strands, which may, by means of a cylindrical commutator, be coupled up in "parallel " or in "series," as it is desired or not to magnify the deflection 10 - fold for small currents. In cases where loss of magnetism in the permanent magnet is to be feared, a spiral spring is made to take the place of the horse - shoe magnet as the controlling force. Again, where extreme delicacy is required, the deflections of the needle may be magnified 10 - fold or more, by attaching to the shaft of the needle a wheel and pinion.

This form of instrument is adapted for use with strong currents.

(5) Deprez's Galvanometer consists of a horse - shoe permanent magnet, with a solid armature - needle of peculiar "fish-bone" form, and index - pointer, pivoted vertically, placed between its poles, and a coil of the wire, which carries the current to be measured, laid within the horse - shoe. A table indicating the value of the several deflections of the needle has to be Used.

(6) Thomson's Current Galvanometer

This instrument, together with its fellow one, for the measurement of the difference of potential, have been termed "graded galvanometers." They are designed to permit of a very wide range being obtained with each respectively; this being effected by varying the intensity of the magnetic field in a known ratio, and comparing with it the unknown current. The present instrument consists essentially of 2 parts; a coil of thick wire, through which the current to be measured is allowed to pass, it being fixed upright at the end of a wooden platform, with about 1/2 only of the coil projecting above it; and a "magnetometer." This last consists of 4 short steel magnets (constituting the "needle") mounted in a frame on a vertical axis, with a long aluminium pointer attached, and enclosed in a quadrant - shaped box, with a glass cover and a silvered glass mirror bottom (in order to counteract any effect of parallax while reading the scale of tangents, to which the instrument is graduated). To increase the directive force of the needle, when required, a semicircular permanent magnet of hard steel, and of known intensity, may be fixed upright on 2 arms projecting from the magnetometer, behind the apex of the quadrant.

The magnet stands over the needle, with its magnetic axis in the horizontal plane through that of the needle. The magnetometer is made to slide along a groove in the wooden platform in front of the fixed coil, and in a direction at right angles to it, the axis of the magnet and of the needle (when at rest) being parallel to that of the coil; while the aluminium pointer, when at zero, which is the centre of the scale, lies in the same direction as the groove, or at right angles to the coil. The range of sensibility obtained by varying the extreme distance along the groove of the magnetometer from the coil is about 50 - fold; while by removing the magnet, and leaving the needle under the influence of the earth's force alone, a further sensibility 50 times greater can be obtained.

(6) Pressure Measurers

(1) Ayrton and Perry's Volt - meter is in principle and form like their am - meter, and has, like it, several similar modifications. The essential difference between the 2 instruments is that the thick wire coil of the am - meter is here replaced by a thin wire coil, with the connections to its ends so arranged as to measure the difference of potential between the points desired.

(2) Thomson's Potential Galvanometer differs only from his "current" instrument, already described, in the use of a thin wire coil (of German silver of about 6000 ohms resistance), instead of a thick wire coil; and in the change in the connections to suit the present object.

(3) Siemens's Torsion Galvanometer consists of a bell - magnet suspended between 2 coils of fine wire by means of a thread and a spiral spring. The deflections of the magnet under the current are counteracted by means of the spring, as in the electro - dynamometer, only that the angles of torsion are here proportional simply to the intensity. Certain resistances can also be added by which the sensitiveness can, by means of the insertion of a plug, be increased 10 - fold.

(C) Power Or Energy Measurers

(1) Ayrton and Perry's Power - meter consists of a thick wire coil through which the main current passes, and of a fine wire coil on a shunt, which is suspended within it. The axes of the 2 coils are parallel, and the passage of a current tends to deflect the suspended one. Since the thick wire coil measures the intensity of the current, and the thin wire one its electromotive force, and as the deflection of the latter is the product of the two, therefore the amount of that deflection is the measure of the power, or energy, of the current. Modi - ficatio s of this instrument, with or without a commutator, or with a wheel and opinion arrangement, are also made.

(2) Siemens's Watt - meter is constructed on the same principle as the electro - dynamometer. The 2 coils are, however, kept distinct, with separate terminals to each. The fixed one, of thin wire on a shunt, measures the electromotive force; while the movable one has the main current passing through it. The resulting position of the latter coil is due directly to the intensities of the 2 currents; that is, to the power developed. This product, of volts x amperes, is watts; hence the name of the instrument. The angle of torsion of the index - pointer gives the watts directly, and with the knowledge of the particular constant.

(D) Resistance Measurers

Measurements of this class are generally carried out by means of a galvanometer, a set of resistance coils, and a Wheatstone bridge (therewith balancing the unknown resistance by means of one of a known amount).

Ayrton and Perry's Ohm - meter consists of 2 coils: a thick wire one for the main current, and a thin wire one on a shunt for the electromotive force. Both are fixed with their axes at right angles to each other. A needle is acted upon by the currents flowing through each, and as these are at right angles to each other, the resulting deflection represents the ratio F.M.F./Current , i.e. resistance; while in the power - meter the deflection is caused by the product of current X E.M.F. In all the instruments devised by Professors Ayrton and Perry, the deflections are caused, by duly proportioning the parts, to be directly proportional to the measurements sought for in each case.