40 o SCIENTIFIC RECORD FOR 18S2. 



turn round a vertical axis. Each magnet is l'^™ long, made of glass- 

 hardened steel wire, No. 18. Four of these form the needle, and the 

 sides of the mounting are prolonged so as to form an index, moving 

 over a graduated scale, the whole inclosed in a quadrautal shaped box. 

 A semicircular magnet placed vertically over the angle of the quad- 

 rant intensifies the field when necessary. The magnetometer box is 

 placed on the platform so that the axis of the needle passes through 

 the center of the coil and can be moved to and from it at pleasure. 

 The coil of the current galvanometer is made of stout copper strip 

 1.2'^'" broad, 1.5*^™ thick, wound in six turns and insulated with as- 

 bestus paper. It will measure currents up to 100 iunperes; and with 

 a single coil of still heavier copper, currents of 1,000 amperes. {Na- 

 ture, September, xxvi, p. 506.) 



Gray has published a valuable article upon the graduation of gal- 

 vanometers for the measurement of currents and potentials in absolute 

 measure, in which he discusses the method of determining the horizon- 

 tal component of the^arth's magnetism, the theory and construction of 

 the standard galvanometer, the theory and relations of electric units, 

 and the method of graduating and using Sir William Thomson's graded 

 galvanometers. (Nature, xxvii, pp. 32, 105, 319, 339.) 



Boys has suggested a current meter based on a new principle. The 

 rate of a pendulum clock depends on gravity and is proportional to the 

 square root of its strength. That of a watch depends on the strength 

 of the hair-spring and is proportional to the square root of its strength. 

 The force due to an electric current is proportional to the square of the 

 current strength. Hence if a portion of a circuit is capable of vibrating 

 under electromagnetic force, the speed of vibration will be proportional 

 simply to the current strength. If, now, such a contrivance takes the 

 place of the balance of a pendulum clock, the clock will measure elec- 

 tric current instead of time. A meter of this kind has been constructed 

 in which the controlling power depends on iron crescents and solenoids, 

 and in which a portion of the main current is shunted through secondary 

 solenoids giving an impulse at each swing, when the balance is in its 

 neutral position, thus producing no effect on the rate of oscillation. 

 {Nature, February, xxv, p. 355.) 



Wartmann has contrived an apparatus which he calls a "rheolyzer" 

 for varying the strength of a derived current from zero to a maximum, 

 indicating at the same time the ratio of these variations. A graduated 

 metallic ring, round a column carried by a tripod, incloses a thick disk 

 of glass or ebonite resting on six radii of the ring. In the upper sur- 

 face of the disk is a circular trough of mercury receiving two copper 

 electrodes at the bottom, 180^ apart. A cross-bar on the top of the 

 column, on which it turns as an axis, acts as a movable Wheatstone 

 bridge; it has two terminal verniers, and two screws dipping in the 

 mercury. These latter are insulated, but communicate through central 



