SEISMIC METHODS 



803 



with another oscillating circuit to produce a beat frequency, which is 

 amplified and rectified, and then recorded in the conventional manner. 



Fig. 491. — Hot wire resistance seismometer. 

 a, fine wire grid; b, inertia mass; c and c', upper 

 and lower diaphragm supports; d, air chamber 

 ring for diaphragms; d' , spacer ring; /, aluminum 

 instrument case; g, carrying handle; h, cable (8 

 wire); i, water-tight lead in bushing; /, spacers. 



Hot Wire Resistance Seismometer 



The operation of the hot wire resistance seismometer depends on 

 changes in the resistance of a hot wire (about 0.005 mm. diameter) when 

 cooled by an air current which is produced by the motion of the inertia 

 mass relative to the seismometer 

 ' case. Because the changes in re- 

 sistance depend on the motion of 

 the air, the instrument responds to 

 the velocity of the earth's surface 

 at the seismometer station. 



An instrument of this type is 

 illustrated schematically in Figure 

 491. It comprises a mass and dia- 

 phragm so mounted that they con- 

 stitute a "bellows" arrangement, 

 whereby relative motion of the 

 mass and the case causes a move- 

 ment of air past the hot wire grid. 

 Two grids are employed, connected 

 in series. A double diaphragm is employed to provide a good mechanical 

 support for the mass and also to form a chamber for the oil serving as a 

 damping medium. The milliampere current required for heating the grids 

 is supplied by a central battery in the recording truck. By means of a 

 transformer in series with the line to the seismometer, the variations in 

 current are picked up and impressed on the amphfiers for recording in the 

 conventional manner. Usually one or two stages of amplification are more 

 than sufficient. This type of seismometer has been found to be especially 

 suitable for the lower frequencies. (Also, it is better adapted for refraction 

 prospecting than for reflection prospecting.) 



Carbon Button Seismometer 



This is one of the simplest forms of seismometers. The operation 

 of the carbon button or carbon granule type seismometer depends on the 

 changes of resistance which occur in carbon due to pressure varia- 

 tions. The resistance changes produce corresponding changes in current. 

 Usually the carbon button is connected in series with the primary of an 

 impedance matching transformer and a source of current (local dry cell). 

 Variations in primary current cause corresponding variations in potential 

 on the secondary of the transformer. The secondary is connected to the 

 recording galvanometer. This type of seismometer" is very efficient and has 

 a high output. For shallow work it may be used without additional ampli- 



