930 APPENDIX. 



tangent of the permanent deflection is a measure of the conductivity of the 

 coil. 



1487b. Resistance Coil in an annular brass case, and em- 

 bedded in paraffin. The resistance of the revolving coil was 

 compared with that of this coil during the experiments. 



Prof. Clerk Maxwell. 



1487c. Bridge Arrangement used in the above comparisons. 



Prof. Clerk Maxwell. 



1495. Universal Resistance Box. 



Siemens and Halske, Berlin. 



This instrument is intended to measure by simple manipulation the resis- 

 tance of wires, the electromotive force of batteries, and their internal re- 

 sistance. It has three branch resistances, and a complete series of resistances 

 from 1 to 5,000 S. U. 



1496. Complete Bridge. Siemens and Halskc, Berlin. 



This bridge, besides the branch resistances, and variable resistance, in- 

 cludes battery reverser, galvanometer key, and short circulating keys, so 

 arranged that one of the branch resistances or the variable resistance can be 

 cut out instantaneously. 



15O4g. Sinus Inductor. Apparatus for producing an induc- 

 tion current by the rotation of a magnetic disc for determining 

 resistance. L. Waibler, Darmstadt. 



15O6. Siemens' Resistance Unit. 



Siemens and Halskc, Berlin. 



This is a wire resistance equivalent to the resistance offered by a prism of 

 pure mercury one square millimetre in section and one metre long at centi- 

 grade, which is the basis of the Siemens' standard unit. 



15O6a. Normal Mercury Unit, in glass spiral. 



Siemens and ffalskc, Berlin. 



167 la. Small registering Electro-Magnet, capable of 

 supplying 500 signals per second. 



Marcel Deprez, 16, Rue Cassinc, Paris. 



1691. Stock Exchange or Bourse Telegraph Instru- 

 ment. Siemens and Halske, Berlin. 



A battery is employed only at the central station, from which the messages 

 are sent by depressing lettered finger keys on the transmitter, which are so 

 arranged that the letters most commonly occurring are nearest at hand. Only 

 one line wire is required. The receiving instrument prints in Roman type, 

 and has a double type wheel carrying figures and letters, so that by depressing 

 a changing key in the transmitting apparatus letters or figures may be printed 

 at will. The type wheel automatically returns to its zero position after each 

 printed signal, and is therefore always in adjustment. 



