St. John — Wave lengths of Electricity on Iron Wires. 313 



The secondary circuit consists of the long rectangle PQRS 

 which is carefully adjusted to resonance before any other meas- 

 urements are made. 



la. 



Q 



859 QUI 



P 



^M 



R 



S 



For determining the occurrence of resonance and for explor- 

 ing the wires to obtain the wave form, the bolometer as de- 

 scribed by Paalzow and Rubens* was used in connection with 

 Ru bens' f adaptation of it. 



The exploring terminals of the bolometer are shown at P, fig. 

 2«. They consist of two capillary glass tubes set in a frame 

 of wood ? the tubes slide over the wires to be explored, and 

 around each tube is wrapped by a single turn one of the leading 

 wires to the bolometer. Electric oscillations in the secondary 

 circuit cause inductively alternating currents along the leading 

 wires through one arm of a balanced Wheatstone bridge, which 

 forms the bolometer. This arm of the bridge is made of fine 

 iron wire and so arranged that the bridge current and the 

 oscillating currents traverse it without affecting each other. 

 The bridge is thrown out of balance by the increase of resist- 

 ance caused by the heat generated from the alternating cur- 

 rents, and a corresponding throw of the galvanometer is pro- 

 duced. 



To adjust the circuit to resonance the exploring terminals 

 were placed at PS (fig. la.) The induction coil was put in 

 action, and the reading of the bolometer taken for this length 

 of wire, a few centimeters of wire were cut off and the reading 

 again taken. This operation was repeated until a maximum 

 point was passed. The wires were renewed and the operation 

 repeated again and again. A sharp and unmistakable maxi- 

 mum was formed when PQ was 859 cm long. The effect fell 



* Anwendung des bolometrische Princips auf electrische Messungen, Annalen 

 xxxvii, p. 529. 



f Ueber stehende electrische Wellen in Drahten ivnd deren Messungen, Annalen 

 xlii, p. 154. 



