Which of the following acts as a circuit protection device?

A soap bubble, having radius of 1 mm, is blown from a detergent solution having a surface tension of 2.5 $$ \times $$ 10^–2 N/m. The pressure inside the bubble equals at a point Z₀ below the free surface of water in a container. Taking g = 10 m/s², density of water = 10³ kg/m³, the value of Z₀ is :

Two similar thin equi-convex lenses, of focal length f each, are kept coaxially in contact with each other such that the focal length of the combination is F₁. When the space between the two lenses is filled with glycerin (which has the same refractive index (m = 1.5) as that of glass) then the equivalent focal length is F₂. The ratio F₁ : F₂ will be :

In which of the following devices, the eddy current effect is not used?