VIII. MEASUREMENT. 291 



This instrument was sent out for use on the first Keel Sea cable (1859), but 

 did not arrive until after the failure of the cable. It returned dismantled, and 

 was never set in action again. It has been superseded by the siphon re- 

 corder. Made by White and Barr (now James White), Glasgow. 



1446a. One of the First Mirror Galvanometers, made 

 for the reading of messages through submarine cables, and used for 

 that purpose at Newfoundland in 1858. This galvanometer has an 

 arrangement for altering the intensity and direction of the directing 

 force by a double motion of the directing magnet. One for varying 

 its distance from the suspended needle and mirror, the other,giving 

 the magnet a motion in azimuth. This arrangement is still used 

 in the Astatic Mirror Galvanometer for testing sub-marine cables. 



Sir William Thomson. 



1447. Ironclad Marine Galvanometer, used on board the 

 "Great Eastern" in the Atlantic Cable Expedition of 1866, and 

 subsequently by Mr. Willoughby Smith in the Mediterranean and 

 Red Sea. Sir William Thomson. 



This instrument is the first ironclad marine galvanometer, and the first with 

 suspension by stretched silk fibre instead of platinum wire. Made by James 

 White, Glasgow. 



1447a. A Differential Galvanometer, constructed specially 

 for testing the locality and nature of faults in submarine cables. 

 It is the first instrument in which shunts were used for practical 

 electrometric purposes. A shunt is applied to one of the wires of 

 the coil so as to multiply the reading of a rheostat ten times. It 

 also was used for telegraphic reading purposes and for measuring 

 the discharge from cables. It was made in 1858, and was in 

 constant use for many years. 



W. H. Preece, General Post Office, London. 



1447b. Thomson's Mirror Galvanometer, with hinged 

 coils, and Shunt for same. Warden, Muirhead, and Clark. 



1448. Absolute Galvanometer or Magnetic Dynamome- 

 ter. Frederick Guthrie, F.R.S. 



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 suspension from a 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 when a cflrrent 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 current. 



1449. Galvanometer for measuring large currents in definite 

 units. Graduated in Weber's for use with the electric light, &c., 

 and in ounces of silver deposited per hour, for use in electro-plating 

 or in other forms of actual work. John T. Sprague. 



T 2 



