VIII. MEASUREMENT. 337 



1447d. Sir William Thomson's Dead-beat Mirror 

 ** Speaking " Galvanometer for receiving signals on long 

 submarine cables, provided with scale, stand, and lens. The 

 mirror carrying the magnets is confined in a small air-chamber 

 which can be contracted at will. The compressed air acts like a 

 cushion, and " damps " the motion of the mirror, thereby prevent- 

 ing oscillations. Warden, Muirhead, and Clark. 



1448. Absolute Galvanometer or Magnetic Dynamo- 

 meter. Frederick Guthrie. 



A current traverses in succession four spirals embracing soft iron cores. 

 Two of the so formed electro-magnets are fixed and two movable together in 

 a horizontal plane by means of the suspending torsion thread. The spirals 

 are such that the magnets repel one another. If, when no current is passing, 

 a beam of light reflected from a mirror attached to the movable pair falls in 

 a certain place, then r when a current passes, the torsion screw head must be 

 turned so as to force the magnets up to the same distance as before. The 

 repulsion or angular torsion is proportional to the square of the strength of 

 the current. 



1449. Galvanometer for measuring large currents in definite 

 units. Graduated in Weber units for use with the electric light, 

 &c., and in ounces of silver deposited per hour, for use in electro- 

 plating and other forms of actual work. John T. Sprague. 



1450. Patent Universal Galvanometer, indicating current 

 and resistance in definite units of measurement. 



John T. Sprague. 



This galvanometer contains four circuits, having 1, 10, 100, and 1,000 fold 

 degrees of action on the needle, enabling it to be used for large or small 

 currents. The patent dial is graduated to indicate the current in actual units, 

 either the British Association, Weber, or in chemical equivalents. It is also 

 graduated to show the total resistance of the circuit in ohms without the aid 

 of a resistance instrument when used with a Daniell cell. By using a fixed 

 resistance it shows the electromotive force of the circuit in volts. 



1450a. Galvanometer for Projections. 



M. J. Duboscq, Paris. 

 1450b. Galvanometer, for thermo-electric currents. 



Luizard) Paris. 



1451. Hhe Electrometer of Marianini, for observing elec- 

 tric discharges between the atmosphere and the earth. 



Robert James Mann, M.D. 



This instrument was planned by Professor Melsens. It contains a coil of 

 copper wire which is to be made continuous with the system of a lightning 

 rod, or with the earth wire of a telegraph line. When an electric spark 

 passes through the coil a soft iron bar in its interior is magnetised, and a 

 traversing magnetic needle pivoted above the coil is then deflected by it out of 

 the north and south line of the earth's magnetism towards either the east or 

 west. When the interior iron has been magnetised it must be replaced by 

 ii neutral bar before another observation can be made. 



40075. Y 



