Early knowledge of electricity was very basic. The nature of electricity was not known. Scientists arbitrarily labelled charges as positive and negative, and gave electric current a positive sign meaning it was carried by positive charges, from the positive terminal to the negative terminal.
Now we know that in solids, the positive charges are fixed, and that electric current is carried by negatively charged electrons. The original current, wrongly said to be carried by positive charges from the positive terminal to the negative terminal, is called conventional current.
In liquids and gases, positive and negative charges, called ions can be present, and are free to move. In liquids and gases, current can be carried by either, or both.
