SEISMIC METHODS 815 



Certain disadvantages of the instrument may be summarized as follows : 

 In order to obtain sufficient optical amplification of the movement of the 

 strings, a lens of short focal length is utilized. When a large number 

 of strings is required it may be difficult to focus all strings sharply 

 on the record. Occasionally, when excessively strong waves are 

 recorded, the strings collide and may tangle. The black background makes 

 it inconvenient to mark reference points and notes on the record. The 

 chief disadvantage of the string galvanometer arises from the fact that if 

 more than about 10 strings are desired, the construction becomes somewhat 

 cumbersome. 



The low impedance of this type of galvanometer necessitates a matching 

 transformer. However, for geophysical work, the low impedance is not a 

 disadvantage, because proper transformer ratios can be employed to match 

 the output impedance of the amplifying tube with the low input impedance 

 of the galvanometer. 



An Einthoven type of galvanometer element consisting of thirteen 

 strings is shown in Figure 500. The individual strings consist of copper 

 wire 0.00075 inches in diameter. They are separated from adjoining 

 strings by means of accurately located grooves in the upright ivory posts 

 and are soldered to batteries of connecting terminals at both ends. These ter- 

 minals may be adjusted individually by set screws to obtain the desired 

 tension in the strings after they are installed. The natural frequency of 

 the elements is about 500 cycles per second, and the system of which they 

 are a part gives a D.C. deflection of about 2 to 4 millimeters per microwatt 

 at a distance of 1 meter. 



Moving Coil or lyArsonval Type.— In the D'Arsonval type galvanom- 

 eter, a movable loop or coil of wire carrying the current to be recorded 

 is suspended between the poles of a strong magnet. Light is reflected 

 to the photographic recording paper from a small mirror attached to the 

 coil. 



Each trace on the record requires a separate galvanometer unit. Records 

 obtained with a D'Arsonval mirror galvanometer can be distinguished 

 from those obtained with an Einthoven string galvanometer because in the 

 former black traces appear on a white background while in the latter white 

 traces appear on a dark background. 



Figure 501 shows a fifteen element moving coil galvanometer with the 

 cover opened for inspection, but with only two elements in place. In 

 galvanometers of this type the frequency is determined mainly by the 

 torsional stififness of the suspensions and by the moment of inertia of the 

 coil and mirror. 



