Question on while loop to script

Hi guys,

I'm new to ruby and I'm writing simple programs to help my understanding
of ruby. I wrote the program below and I would like to know if you guys
could recommend a while loop to use before the if statement so the
program doesn't end until a key is pressed? I want it to keep running
until the user quits. Also does anyone think I can make the program
better?class OhmsLaw
  def initialize(voltage, current, resistance)
    @voltage = voltage
    @current = current
    @resistance = resistance
  end
  def voltageNil
    @current * resistance
  end
  def currentNil
    @voltage / @resistance
  end
  def resistanceNil
    @voltage / @current
  end
  def to_s
    "String representation of OhmsLaw V: #{@voltage}, I: #{@current}, R:
#{@resistance}"
  end

    #This will make these variables accessoable from outside
    #of the class
  attr_accessor :voltage, :current, :resistance
  #solve4Current can be used for :currentNil as  an alias for it
  alias :solve4Current :currentNil
end

#created a new object called solve that will take V,I,R as inputs
solve = OhmsLaw.new(9.0, nil, 2)
#attr_accessor allows us to change voltage of 9 to 16 volts from
#outside of the class
solve.voltage = 16

if solve.voltage == nil
  puts solve.voltageNil
elsif solve.current == nil
  puts solve.solve4Current
else
  puts solve.resistanceNil
end

puts "#{solve}"

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