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The units in which in, r and c are expressed and the meaning 

 Avhich must be attributed to the words "virtual mass of the image 

 of the string" and "virtual resistance" are dealt with in the following 

 considerations on 



3. The mass of the string. 



In practice it is impossible or scarcely possible to find the mass 

 of the string by direct weighing. For this the weight is too small. 

 Also a calculation of the mass from the dimensions and the specific 

 gravity of quartz and silver does not lead to the desired result, since 

 the measurement of the diameter of the fine thread ajid of the thick- 

 ness of the still much thinner layer of silver caimot be carried out 

 with the necessary accuracy, the more so because the layer of silver 

 often shows inequalities and its thickness is irregular. 



So we must proceed in another way and we reach our purpose 

 in the simplest way if we start from the "virtual mass of the image 

 of the string". The image of the middle of the quartz thread, which 

 is projected with great magnification, moves along a straight line 

 over a millimetre scale each time the string deflects. Instead of this 

 image we imagine a material point, acted on by a force which is 

 equal to the force that moves the whole string. The acceleration 

 experienced by the fictitious material point from this force, determines 

 what we call the "virtual mass of the image of the string". 



The unit in which this virtual mass must be expressed depends 

 of course on the units chosen for the force and the acceleration. As 

 the cause of the motion of the string is the electric current passed 

 through the galvanometer, the moving force can be expressed in 

 units of intensity of current. We choose for this purpose the micrampere. 



The unit of acceleration is determined by the units chosen for the 

 distance and the time. In accordance with the millimetre scale along 

 which the image moves, the distance is expressed in millimetres, 

 Avhile it is found practical to take the millimetre also as the unit 

 of time. For the deflection of the quartz thread is recorded on a 

 plane which is moved along with uniform velocity perpendicularly 

 to the direction of the motion of the image, so that in the recorded 

 curves, from which we must always derive the data for further cal- 

 culation, time is represented by a length. 



The above mentioned units of force, distance and time form a 

 system which may be called the inillimetre-micrampere or [inm — [lA] 

 system. Expressed in this system the unit of virtual mass m is the 

 mass which experiences from a force of I micrampere an acceleration 

 of 1 mm. distance per mm. of time. 



