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n o t e A variable’s monitor also indicates its scope. If a variable belongs to one sprite, its
monitor should show the sprite name before the variable name. For example, the moni-
tor Cat speed 0 indicates that speed belongs to Cat. If the speed variable were a
global variable, its monitor would only say speed 0. The difference between the two
cases is illustrated in the following figure.
This monitor indicates that the speed
variable belongs to the Cat sprite.

This monitor indicates that speed is
a global variable.

using Variable monitors in applications

Now that you know the basics behind variable monitors, I’ll show you some
ways you could use them to add some extra functionality to your Scratch
applications.
The ability to use monitors as both displays and controls opens the
door for a wide range of applications, including games, simulations, and
interactive programs. Let’s explore some examples that make use of moni-
tors in the following subsections.

Simulating Ohm’s Law
OhmsLaw .sb2 Our first example is a simulation of Ohm’s law. When a voltage (V) is
applied across a resistor (R), a current (I) will flow through that resistor.
According to Ohm’s law, the amount of current is given by this equation:

VoltageV ()
CurrentI () =
Resistance R ()

Our application allows the user to change the values of V and R using
slider controls. Then it calculates and displays the corresponding value of
the current, I. The user interface for this application is shown in Figure 5-21.

Current The Light sprite
monitor changes the brightness
of the bulb.

Voltage
control Resistance
control Three sprites whose
sizes show the relation
between V, I, and R.

Figure 5-21: User interface for the Ohm’s law application

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