V.3 No 1

79

Several experiments studying dynamic magnetic field

3.2. Description of experimental device

To check this peculiarity, we used the ferromagnetic core with a plane gap = 3 mm  at the central leg, as it is shown in Fig. 11.

Fig. 11. Experimental device checking the process of emf induction in a single conductor in a local homogeneous magnetic field

The cross-section of the gap is 32 60 mm. At the side legs of core we put two identical windings w₁ = w₁ = 3000   turns each, winded by the wire with the diameter 0,23 mm, with one layer of capacitor paper every 500 turns as an insulation between the layers. The coils were phased so that the field produced in the gap combined - it means, in parallel connection the start of one winding was connected with the start of that another. Into the central gap we have put the leg of special frame consisting of 10 loops made of wire 0,23 mm in diameter; they were fixed on the textolite frame, as it is shown in Fig. 12. We soldered each loop at the point B. All 10 loops were connected in series. The start of each next loop (input A) was connected with the output of previous (the point A' ). The size of frame we chose so that it considerably exceeded the size of core, and the more the size of gap. The overall size of frame was 130 240 mm .

Such design of frame has been chosen in order to compensate the supposed parasitic emf able to arise when the probe leg has been put into the gap of core. The further study with different kinds of frames showed this precaution excessive.

Fig. 12. Design of probe frame for the experiment studying the process of induction in a single wire

To diminish the parasitic affection of the primary winding onto the bypass wires of frame, we conducted the experiment at low frequency - 50 Hz. The electric circuit of the experimental device is shown in Fig. 13.

Fig. 13. The electric circuit of the experimental device for studying the process of induction in a single wire

The circuit consisted of autotransformer 1 used to vary the voltage across the primary windings. To the output of the autotransformer we connected the primary windings 2 connected in parallel. We picked the signal off the frame 3 and by the amplifier having k_amp = 200 and R_in = 1 MOhm   we applied it to the Y input of oscillograph whose start was synchronised with the voltage across the primary windings 2.

When preparing the experiment, we additionally have checked, how the parasitic emf affected the tapping parts of frame. To do so, we have put into the device the frame without the central leg. We have revealed that the parasitic affection is at the level of noise and practically does not effect on the value of measured emf induced in the probe.