190 KNOBLAUCH ON RADIANT HEAT. 



enters and passes from both portions, and thus passes double 

 through half the length. 



The combined magnetic needles are 7'0 centim. in length, 

 I'l millim. diameter in the centre, and 17'0 millim. distant from 

 each other. The little ivory column which supports them is 

 suspended from the finest possible silk-worm thread 30 centim. 

 in length. Thus they form a system, which completes a simple 

 oscillation in sixteen seconds, and assumes an almost constant 

 position, determined by the torsion of the thread and the com- 

 bined action of the magnetism of the needles*, of about 45° 

 towards the magnetic meridian. The upper needle vibrates 

 above a circular disc of copper precipitated by galvanism, of 

 8 centim. 5"0 milhm. in diameter, and cut through at an angle 

 of 90°, and upon the silvered margin of which the graduated 

 division of the circle is engraved. 



A cylindrical glass case, 6 centim. high and 14 centim. in 

 diameter, surrounds the whole ; and its upper plate being only 

 1 centim. fi'om the copper disc, admits of our reading off to half 

 a degree. In its centre a glass tube 32*5 centim. high and 22*4 

 millim, in diameter is placed ; this surrounds the silk thread, 

 which is fixed to a metallic rod in its upper part, 14 centim. in 

 length, capable of moving in a vertical direction, and serving at 

 the same time to stop the motion of the needles. 



To protect the instrument from vibrations, it was placed upon 

 a bracket, which was fastened to the wall by brass nails. The 

 wires which united it to the thermal pile are not screwed imme- 

 diately into the galvanometer, but to separate solid copper 

 sockets, which are in firm union with the extremities of the coils. 



The thermal pile and multiplier wei'e made by M. Kleiner, 

 one of the most skilful mechanics in Berlin. I convinced my- 

 self by experiment, that on using a pile of twenty-five pairs it 

 would be more advantageous to pass the current through the en- 

 tire length of the galvanometer wire, than to conduct it in two 

 portions through half the length ; for the same source of heat 

 which in the first case caused a deviation of 28° in the magnetic 

 needle, in the second combination of the wire caused a deflection 

 of only 26°*5 ; oi in the first case of 37°, in the second of 35° ; or 

 in the first of 51°, and the second of 48°. 



The former mode of closing the circuit has therefore been used 



throughout the series of experiments f. 



• Moser, Dove's Repert. vol. i. p. 2G0. 



t That exactly the reverse should occur with a single pair, in which the 



ii 



