For the duration of the sparks in the chamber (see Why does the spark chamber spark), a large but short-lived current exists which produces a magnetic field, which in turn induces an electric field near to the plates. These fields are unable to die away (due to Maxwell's fourth equation for fields which vary rapidly with time, the equation is omitted here for simplicity as it is unimportant in the following discussion); instead the changing electric and magnetic fields combine in such a way as to produce new fields further out from the plates as the old ones die away. This process is continually repeated to give a combination of related electric and magnetic fields travelling outwards from the plates.
The current within the spark has associated with it energy, some of which is dissipated by moving out with the radiated fields. The radiated fields become fundamentally different as distance from the source is increased. In the region close to the spark chamber the fields are known as 'near fields', further from the chamber the fields are known as 'far fields'.
The far field is the simpler of the two where the electric (E) and magnetic (H) components of the field always have the same relative amplitude (The ratio E/H is known as the 'Impedence of free space' and has a value of 377 Ohms)
The near field region is the area of interest. Experimentation has shown that the spark chamber is a high impedence source which has the characteristic that it generates mostly electric fields within the near field region.
These radiated fields are unwanted as they are capable of interfering with the operation of other equipment. This interference is known as electromagnetic interference (EMI). EMI has been eliminated from the transportable spark chamber by implementing a shield over the detector part of the chamber:
Figure 1: Metal shield over the detector part of the spark chamber, implemented so as to eliminate EMI
The shield works by attenuating and reflecting the signal as it tries to pass through the shield. Reflectivity is the most important of these two when considering high impedence sources, so a shielding material of high conductivity was important.
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