Fill in the blanks:
$(a)$. Longer line in the symbol for a cell represents its______terminal.
$(b)$. The combination of two or more cells is called a_________ .
$(c)$. When current is switched ‘on’ in a room heater, it________ .
$(d)$. The safety device based on the heating effect of electric current is called a_________.

$(a)$. Longer line in the symbol for a cell represents its $\underline{positive}$ terminal.

Explanation:

There are two terminals$(positive\ and\ negative)$ in an electric cell. A cell is represented as shown in the figure below. The longer line in the symbol represents the positive terminal of the battery.

$(b)$. The combination of two or more cells is called a $\underline{battery}$.

Explanation:

When two or more electric cells are connected together, this combination is called a battery of cells.

$(c)$. When current is switched ‘on’ in a room heater, it $\underline{produce\ heat}$.

Explanation:

A room heater is a device that converts electrical energy into heat. When the current is passed through the circuit of a room heater, then it starts producing heat. This is called the heating effect of electric current.

$(d)$. The safety device based on the heating effect of electric current is called a $\underline{fuse}$. 

Explanation:

A fuse wire is made of metals that have a low melting point, due to which whenever a slightly higher electric current flows through it, it breaks the wire and hence damage to electrical appliances is prevented. It is based on the heating effect of electric current.